Dissertations / Theses on the topic 'Small field dosimetry'
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1
Charles, Paul Henry. "Very small field dosimetry." Thesis, Queensland University of Technology, 2014. https://eprints.qut.edu.au/78916/4/Paul_Charles_Thesis.pdf.
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This thesis investigated in detail the physics of small X-ray fields used in radiotherapy treatments. Because of this work, the ability to accurately measure dose from these very small X-ray fields has been improved in several ways. These include scientifically quantifying when highly accurate measurements are required by introducing the concept of a very small field, and by the invention of a new detector that responds the same in very small fields as in normal fields.
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Cranmer-Sargison, Gavin. "Small field dosimetry : experimental methods and Monte Carlo simulation in small field radiation therapy dosimetry." Thesis, University of Leeds, 2014. http://etheses.whiterose.ac.uk/7762/.
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The goal of the thesis was to investigate, and better define, what the requirements are for accurate small field relative dosimetry. Diode detector selection and experimental techniques were evaluated. EGSnrc Monte Carlo simulations were used to predict diode detector dosimetric parameters and assist in interpreting measured data. An emerging scintillator based detector technology was also tested and methods developed to standardize the reporting of small field dosimetric data. Using careful experimental methods the relative output uncertainty for the smallest square field size of side 0.5 cm was reduced to better than ±1.00% for all detector types. Monte Carlo simulation data revealed that for the same small field size the relative output measured using unshielded and shielded diodes will be 5% and 10% greater than the actual relative output in water. Further simulation work showed that simplified diode detector models are valid for use in small field dosimetry simulations. The diode detector over-response was also shown to be insensitive to variations in the electron energy and spot size incident on the Bremsstrahlung target. Experimental methods were refined to include the definition of an effective field size, which was shown to remove much of the ambiguity in reporting small field relative output data across a population of linear accelerators. Each of the for mentioned areas of investigation have been shown to be requirements for accurate small field relative.
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Wong, Christopher James, and chrisjwong@yahoo com au. "High Resolution Polymer Gel Dosimetry for Small and Micro Field Dosimetry, and Development of Innovative Polymer Gel Dosimeters." RMIT University. Medical Sciences, 2009. http://adt.lib.rmit.edu.au/adt/public/adt-VIT20091002.161512.
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Current radiotherapy techniques are focused on delivering effective treatments while sparing surrounding healthy tissues. As a result, radiotherapy treatments are using narrower and more tightly conforming therapy techniques. For these treatments to be effective an accurate measure of the dose delivered by these very narrow radiotherapy beams, both in and around the target volume, is required. It is a challenging task for the conventional type dosimeters to determine dose distribution in such small fields. The best example of such fields is microbeam beam radiotherapy (MRT), a developing treatment technique that takes this requirement even further. MRT delivers an array of micrometre size radiotherapy beams to the target. MRT has been shown to be highly effective, but reliable dosimetry of MRT is challenging due to the micrometre scales involved. Attempts to determine the MRT dose distribution have been documented for using special type dosimeters such as radioch romic film and MOSFET detectors, as well as Monte Carlo simulations. This thesis investigates polymer gels as a dosimeter for dose distribution measurements of small radiotherapy fields and microbeams. Polymer gel dosimetry is a technique which uses a tissue-equivalent gel to act as both a three-dimensional dosimeter and a phantom at the same time. These gels polymerise when exposed to ionising radiation and the response is locally dose dependent linearly. This thesis investigates the use of polymer gels for the dosimetry of small sub-centimetre (down to 3 × 3 mm2) and micrometre radiotherapy fields. A high resolution imaging technique is also required for such small beam dosimetry. This work used special high strength MRI scanners to analyse polymer gels at high resolution. This work explores the feasibility of polymer gels irradiated by microbeams and analysed using Raman spectroscopy as a dosimeter for synchrotron generated microbeams. MRT is characterised by very high doses, and special high-dose resistive types of gel were developed as part of this work. It is shown that polymer gels imagined using Raman spectroscopy techniques are capable of measuring the dose distribution of microbeam radiation techniques. This thesis also investigates the use of polymer gels to measure dose perturbations caused by metallic artefacts. Metallic artefacts, such as a surgical aneurysm clip, can be left in a patient and cause dose perturbations during radiotherapy procedures. Polymer gels were used to determine the degree of dose enhancement induced by an aneurysm clip placed inside when irradiated with a typical stereotactic radiotherapy procedure. In addition, this thesis used gels in several other innovative applications. Photonuclear interactions generated in gel by high energy x-ray beams were measured via secondary neutrons. Special clear-type gels that do not change colour with irradiation were developed. Polymer gels were investigated for dosimetry of an extremely high dose rate capacitor type linear accelerator. And polymer gels were for measurement of a high energy proton beam.
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Qureshi, Aleem. "A Novel Equivalent Squares Formalism for use in Small Field Dosimetry." University of Toledo Health Science Campus / OhioLINK, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=mco149338511600157.
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Underwood, Ryan John. "Small field dose measurements with Gafchromic film." Thesis, Georgia Institute of Technology, 2013. http://hdl.handle.net/1853/47675.
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Purpose: To examine the dosimetric characteristics of Gafchromic EBT3 film when measuring small fields of radiation, and compare it against other common radiation detectors.
Methods and Materials: EBT3 film was placed in a solid water phantom and irradiated with 6MV photons, field sizes from 10x10cm2 down to 6x6mm2. The films were scanned with a Vidar DosimetryPRO Advantage Red scanner, and analyzed with RIT113 software. The films were also scanned at different orientations and times to quantify the discrepancies associated with scanning orientation and post-exposure darkening. The same fields were measured with a PTW TN30013 farmer chamber, an Exradin T1 cylindrical ion chamber, a PTW parallel plate ion chamber, and a Sun Nuclear Edge Detector (diode). Output factors were calculated for each detector and compared for accuracy. The output factors were measured from a Varian Clinac iX, Clinac 21EX, Trilogy, and TrueBeam; as well as a Novalis Tx. The outputs from different machines at different clinics were compared.
Results: The EBT3 film and Edge Detector were the only detectors that succeeded in accurately measuring the output from all field sizes; the ion chambers were too large and failed for field sizes below 4x4cm2 due to volume averaging. The dose measured with the film increased by an average of 8.8% after one week post-irradiation. The dose measured was also reduced by an average of 4.4% by scanning the film in landscape orientation, as opposed to portrait orientation. It was shown that the output factors for the smallest field of 6x6mm2--successfully measured with film and diode--varied between 0.54-0.74 for five different machines at three different clinics.
Conclusions: The feasibility of using Gafchromic EBT3 film to measure very small fields of radiation is confirmed. Of the other 4 detectors used, only the diode was shown to be capable of accurately measuring small fields of radiation. The need to optimize the film dosimetry process--including the time films are scanned post-irradiation, the consistency of the scanning orientation of the calibration and subsequent films, and the measurement procedure on the computer software--is highlighted.
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Polston, Gregory K. "A dosimetric model for small-field electron radiation therapy." Muncie, Ind. : Ball State University, 2008. http://cardinalscholar.bsu.edu/366.
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Martínez, Rovira Immaculada. "Monte Carlo and experimental small-field dosimetry applied to spatially fractionated synchrotron radiotherapy techniques." Doctoral thesis, Universitat Politècnica de Catalunya, 2012. http://hdl.handle.net/10803/81470.
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Two innovative radiotherapy (RT) approaches are under development at the ID17 Biomedical
Beamline of the European Synchrotron Radiation Facility (ESRF): microbeam radiation
therapy (MRT) and minibeam radiation therapy (MBRT). The two main distinct characteristics
with respect to conventional RT are the use of submillimetric field sizes and spatial
fractionation of the dose. This PhD work deals with different features related to small-field
dosimetry involved in these techniques. Monte Carlo (MC) calculations and several experimental
methods are used with this aim in mind. The core of this PhD Thesis consisted of the
development and benchmarking of an MC-based computation engine for a treatment planning
system devoted to MRT within the framework of the preparation of forthcoming MRT
clinical trials. Additional achievements were the definition of safe MRT irradiation protocols,
the assessment of scatter factors in MRT, the further improvement of the MRT therapeutic
index by injecting a contrast agent into the tumour and the definition of a dosimetry protocol
for preclinical trials in MBRT.
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Puxeu, Vaqué Josep. "Contributions to new challenges in radiotherapy : from small field dosimetry to stereotactic body radiation therapy." Doctoral thesis, Universitat Politècnica de Catalunya, 2015. http://hdl.handle.net/10803/663216.
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The use of small Fields has substantially increased in radiotherapy during recent years mainly due to expansion in the use of modulated techniques which are also used at some institutions for Stereotactic Body Radiotherapy (SBRT). The use of modulated techniques in lung SBRT is controversial due to the interplay effect. At the same time, small-volume dosemeters have appeared on the market in an attempt to try to cover new dosimetry necessities.
This PhD deals with different features related to small-field dosimetry. It first analyzes the influence of differences between ionization chambers of the same type on dosimetry factors determining 0.5% as the upper limit on the possible influence. Correction factors for PTW-31016 are also proposed and the effect of a particular chamber of this model has been found to be up to 2.8% for 0.5 cm square. These correction factors have been determined by Monte Carlo calculations and have been validated with measurements on fattened and unflattened beams by analyzing the main sources of influence. The last chapter is devoted to SBRT, and suggests a fast and accurate plan set-up together with PUMA, a method to evaluate the interplay effect in lung SBRT.L’ús de camps petits de radiació s’ha incrementat molt durant els últims anys, degut a l’expansió en l’ús de tècniques d’intensitat modulada. Aquestes tècniques també s’utilitzen en alguns centres per irradiar pacients en el que es coneix com a SBRT (Stereotactic Body Radiotherapy). L’ús de tècniques d’intensitat modulada en SBRT de pulmó és controvertit degut a L’interlay effect. En paral·lel, en els darrers anys han aparegut nous dosímetres de petit volum per intentar cobrir les noves necessitats en dosimetria. Aquesta tesi s'ocupa de diferents característiques relacionades amb la dosimetria dels camps petits. En primer lloc, s'analitza la influència de les diferències entre les càmeres d'ionització del mateix tipus en els diferents factors relacionats amb la dosimetria, i s’observa que en condicions de referència les diferencies en els factors de correcció són inferiors al 0.5%. També es proposen factors de correcció per volum per la càmera d’ionització PTW-31016, Es determina que les petites diferencies en la geometria poden comportar diferències en els factors de correcció de fins a 2.8% per a un camp quadrat de 0.5 cm. Aquests factors de correcció han estat determinats mitjançant simulacions Monte Carlo i s'han validat en feixos de fotons amb i sense filtre aplanador. Al mateix temps s’ha fet un anàlisi de les principals fonts d'influència. L'últim capítol està dedicat a la SBRT, on es proposa una configuració per irradiar en poc temps un pla de tractament de SBRT juntament amb PUMA, un mètode per avaluar l’interplay effect en SBRT de pulmó
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Scott, Alison Jane Dalgleish. "Monte Carlo modelling of small field dosimetry : non-ideal detectors, electronic disequilibrium and source occlusion." Thesis, University of Liverpool, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.533947.
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Morales, Johnny Estuardo. "Advances in very small x-ray field dosimetry for circular cones used in stereotactic radiosurgery." Thesis, Queensland University of Technology, 2019. https://eprints.qut.edu.au/133137/1/Johnny_Morales_Thesis.pdf.
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This project presents recent advances in small field dosimetry for radiosurgery treatments with Brainlab circular cones using 6 MV x-rays. The specific advances included: the use of a new microDiamond detector, the use of radiochromic film and OSLD detectors with an extrapolation technique, skin dose measurements and Monte Carlo simulation of radiosurgery treatments. There is limited published data on the output factor for the 4 mm circular cone. This thesis provides methodology and guidelines on how to perform such difficult measurements.
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Courter, Erik Joseph-Leonard. "Use of ClearView Gel Dosimeter for Quality Assurance and Testing of Stereotactic Radiosurgery." Wright State University / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=wright1466417256.
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Omar, Artur. "Silicon Diode Dose Response Correction in Small Photon Fields." Thesis, Stockholm University, Medical Radiation Physics (together with KI), 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-42649.
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Silicon diodes compared to other types of dosimeters have several attractive properties, such as an excellent spatial resolution, a high sensitivity, and clinically practical to use. These properties make silicon diodes a preferred dosimeter for relative dosimetry for several types of measurements in small field dosimetry, e.g., stereotactic treatments and intensity modulated radiotherapy (IMRT). Silicon diodes are, however, limited by an energy dependent response variation in photon beams, resulting in that the diode readout per dose to the phantom medium varies with photon spectral changes, thereby introducing a significant uncertainty in the measured data. The traditional solution for the energy dependent over-response caused by low-energy photons is to use diodes with a shielding filter of high atomic number. These shielded diodes, however, show an incorrect readout for small fields due to electrons scattered from the shielding (Griessbach et al. 2005). In regions with degraded lateral electron equilibrium (LEE) shielded diodes over-respond due to an increased degree of LEE, as a consequence of the high density shielding (Lee et al. 2002).
In this work a prototype software that corrects for the energy dependent response of a silicon diode is developed and validated for small field sizes. The developed software is based on the novel concept of Monte Carlo (MC) simulated fluence pencil beam kernels to calculate spectra (Eklund and Ahnesjö 2008), and the spectra based silicon diode response model proposed by Eklund and Ahnesjö (2009). The software was also extended to include correction of ionization chambers, for the energy dependent Spencer-Attix water/air stopping power ratio (sw,air). The calculated sw,air are shown to be in excellent agreement with published values to better than 0.1% for most values, the maximum deviation being 0.3%.
Measured relative depth doses, relative profiles, and output factors in water, for small square field sizes, for 6 MV and 15 MV clinical photon beams are presented in this work. The results show that the unshielded Scanditronix-Wellhöfer EFD3G silicon diode response, corrected by the developed software, is in excellent agreement with reference ionization chamber measurements (corrected for change in sw,air), the maximum deviation being 0.4%.
Measurements with two types of shielded diodes, namely Scanditronix-Wellhöfer PFD silicon diodes (FP1990 and FP2730), are also included in this work. The shielded diodes are shown to have an over-response as large as 2-3.5% for field sizes smaller than 5 cm x 5 cm. The presented results also suggest a difference in accuracy as large as 0.5-1% between the two types of shielded diodes, where the spectral composition at the measurement position dictates which type of diode is more accurate.
The fast correction of silicon diodes provided by the developed software is more accurate than shielded diodes for small field sizes, and can in radiotherapeutic clinical practice increase the dosimetric accuracy of silicon diodes.
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Wong, Tony Po Yin, and tony wong@swedish org. "Improving Treatment Dose Accuracy in Radiation Therapy." RMIT University. Applied Sciences, 2007. http://adt.lib.rmit.edu.au/adt/public/adt-VIT20080104.144139.
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The thesis aims to improve treatment dose accuracy in brachytherapy using a high dose rate (HDR) Ir-192 stepping source and in external beam therapy using intensity modulated radiation therapy (IMRT). For HDR brachytherapy, this has been achieved by investigating dose errors in the near field and the transit dose of the HDR brachytherapy stepping source. For IMRT, this study investigates the volume effect of detectors in the dosimetry of small fields, and the clinical implementation and dosimetric verification of a 6MV photon beam for IMRT. For the study of dose errors in the near field of an HDR brachytherapy stepping source, the dose rate at point P at 0.25 cm in water from the transverse bisector of a straight catheter was calculated with Monte Carlo code MCNP 4.A. The Monte Carlo (MC) results were used to compare with the results calculated with the Nucletron Brachytherapy Planning System (BPS) formalism. Using the MC calculated radial dose function and anisotropy function with the BPS formalism, 1% dose calculation accuracy can be achieved even in the near field with negligible extra demand on computation time. A video method was used to analyse the entrance, exit and the inter-dwell transit speed of the HDR stepping source for different path lengths and step sizes ranging from 2.5 mm to 995 mm. The transit speeds were found to be ranging from 54 to 467 mm/s. The results also show that the manufacturer has attempted to compensate for the effects of inter-dwell transit dose by reducing the actual dwell time of the source. A well-type chamber was used to determine the transit doses. Most of the measured dose differences between stationary and stationary plus inter-dwell source movement were within 2%. The small-field dosimetry study investigates the effect of detector size in the dosimetry of small fields and steep dose gradients with a particular emphasis on IMRT measurements. Due to the finite size of the detector, local discrepancies of more than 10 % are found between calculated cross profiles of intensity modulated beams and intensity modulated profiles measured with film. A method to correct for the spatial response of finite sized detectors and to obtain the
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Baluti, Florentina. "Monte Carlo Simulations of Chemical Vapour Deposition Diamond Detectors." Thesis, University of Canterbury. Physics and Astronomy, 2009. http://hdl.handle.net/10092/3190.
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Chemical Vapour Deposition (CVD) diamond detectors were modelled for dosimetry of radiotherapy beams. This was achieved by employing the EGSnrc Monte Carlo (MC) method to investigate certain properties of the detector, such as size, shape and electrode materials. Simulations were carried out for a broad 6 MV photon beam, and water phantoms with both uniform and non-uniform voxel dimensions. A number of critical MC parameters were investigated for the development of a model that can simulate very small voxels. For a given number of histories (100 million), combinations of the following parameters were analyzed: cross section data, boundary crossing algorithm and the HOWFARLESS option, with the rest of the transport parameters being kept at default values. The MC model obtained with the optimized parameters was successfully validated against published data for a 1.25 MeV photon beam and CVD diamond detector with silver/carbon/silver structure with thicknesses of 0.07/0.2/0.07 cm for the electrode/detector/electrode, respectively. The interface phenomena were investigated for a 6 MV beam by simulating different electrode materials: aluminium, silver, copper and gold for perpendicular and parallel detector orientation with regards to the beam. The smallest interface phenomena were observed for parallel detector orientation with electrodes made of the lowest atomic number material, which was aluminium. The simulated percentage depth dose and beam profiles were compared with experimental data. The best agreement between simulation and measurement was achieved for the detector in parallel orientation and aluminium electrodes, with differences of approximately 1%. In summary, investigations related to the CVD diamond detector modelling revealed that the EGSnrc MC code is suitable for simulation of small size detectors. The simulation results are in good agreement with experimental data and the model can now be used to assist with the design and construction of prototype diamond detectors for clinical dosimetry. Future work will include investigating the detector response for different energies, small field sizes, different orientations other than perpendicular and parallel to the beam, and the influence of each electrode on the absorbed dose.
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Paskalev, Kamen A. "Dosimetry of very small photon fields." Thesis, McGill University, 2002. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=29463.
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Several dosimetric parameters were measured for three very small radiation fields (1.5, 3, and 5 mm diameter at the machine isocenter) with a small ionization chamber and a new type of radiochromic film. The experiments were carried out on a Clinac-18 linac and the fields were shaped by specially manufactured collimators. When measuring dose profiles, the ionization chamber measurements were first corrected for response variation in off-axis direction, and then deconvolved to eliminate the blur due to the poor resolution of the chamber. The measured data agreed with Monte Carlo simulations within the established statistical uncertainties.Dynamic stereotactic radiosurgery was carried out on the same accelerator using the very small radiation beams. The dose distributions and their displacements from the laser-defined isocenter of the linac were measured and then compared to 3-D Monte Carlo calculations. The results proved that dynamic radiosurgery with very small beams has potential for clinical use.
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Piliero, Maria Antonietta. "Modelling and development of tissue-equivalent dosimeters for small field radiotherapy." Thesis, Swansea University, 2013. https://cronfa.swan.ac.uk/Record/cronfa42465.
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A radiotherapy treatment is a clinical treatment which makes use of ionizing radiation to treat cancerous diseases. However, the ionizing radiation interacting within the cells can lead to DNA damage in both the cancerous and normal tissues. Therefore the exact knowledge of the dose delivered to the patient is essential because it greatly affects the effectiveness of the treatment. Dosimetry is usually performed by air ionization chambers however their use in the dosimetry of small photon beams is limited by their large sensitive volume. The ideal detector has a small, water-equivalent sensitive volume but the design of the detector and the presence of the encapsulation materials placed in close proximity to the sensitive volume can cause perturbations to the radiation fluence. The Monte Carlo method is the ideal tool because it allows a detailed investigation of the perturbation effects of each detector component but a Monte Carlo model often requires detailed information of the device which can be difficult to access. In this study, an experimental approach involving the use of CT scans and fluorescence spectroscopy in the measurements of the physical properties of a liquid ion chamber was explored. The performance of eight single crystal CVD diamond detectors in the dosimetry of photon beams was also assessed. One of the drawbacks of diamond detectors is the dose rate dependence. The evaluation of the dose rate dependence using clinical photon beams is controversial because the dose rate can be varied by either changing the source to detector distance or the Pulse Repetition Frequency of the LINAC machine. A simple analytical model of the charge collection dynamics was written in the Matlab code to understand the effects introduced by a pulsed radiation beam. The outcome of this study correlates the PRF dependence with the presence of deeper traps.
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Sabino, Talita. "Estudo e validação da dosimetria em condições de não-referência." Universidade de São Paulo, 2011. http://www.teses.usp.br/teses/disponiveis/85/85131/tde-25082011-105058/.
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Com o avanço tecnológico dos equipamentos utilizados em radioterapia tornaram-se possíveis o uso de alguns campos de radiação nomeados como campos pequenos em algumas modalidades especiais de radioterapia. Com isso, a dosimetria dos feixes de radiação também teve de ser revista, pois com estes novos tamanhos de campos, não há condição de referência como aquela dita nos protocolos e diretrizes de dosimetria. Neste trabalho foi realizado um estudo completo sobre os campos pequenos, bem como sua dosimetria, mostrando o comportamento dos detectores nessa nova condição de dosimetria através de uma comparação entre os detectores utilizados e dados já publicados na literatura. Além disso, os dados obtidos experimentalmente puderam ser validados através de comparação com dados publicados por outros autores. Na caracterização do detector de diamante o mesmo apresentou-se apropriado em todos os parâmetros para medidas com campos pequenos. Na análise do fator de qualidade Q os resultados experimentais obtidos nesta investigação mostraram diferenças percentuais de 1,8%; 4,0% e 4,9% para câmara do tipo CC01, CC13 e diodo estereotáxico respectivamente. Na avaliação de PDP e TMR foi possível observar a dificuldade nas medições com campos pequenos bem como na comparação de diferentes detectores, para PDP a maior diferença foi de 2,6% e para TMR 2,7%.With the technological equipment used in radiotherapy became possible use of some radiation fields named small fields in some special forms of radiotherapy. Thus, the dosimetry of radiation fields also had to be revised, as with these new sizes of fields, there is no reference condition such as that expressed in dosimetry protocols and guidelines. This work represents a complete study of small fields and its dosimetry, showing the behavior of the detectors in this new condition of dosimetry through a comparison between the detectors used and data already published. Moreover, the experimental data can be validated by comparison with data published by others authors. In the characterization of the same diamond detector has been considered appropriate in all parameters measured with small fields. The analysis of the beam quality factor (Q) the experimental results obtained in this study showed differences in percentages of 1.8%, 4.0% and 4.9% for chamber-type CC01, CC13 and stereotactic diode respectively. In evaluating PDP and TMR was possible to observe the difficulty in measurements with small fields and the comparison of different detectors, the biggest difference for PDP was 2.6% and 2.7% for TMR.
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SABINO, TALITA. "Estudo e validação da dosimetria em condições de não-referência." reponame:Repositório Institucional do IPEN, 2011. http://repositorio.ipen.br:8080/xmlui/handle/123456789/10012.
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Dissertação (Mestrado)
IPEN/D
Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN/SP
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Underwood, Tracy Sarah Amy. "The dosimetry of small, megavoltage photon fields : correction factors, dose area products and detector designs." Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:0266f2b0-93fe-4588-ab42-d7db0f59a3cb.
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In recent years, small fields have come to play a key role in advanced radiotherapy, yet protocols to perform dosimetry under small field conditions are still in their infancy. In 2008, the IAEA and AAPM published a formalism [Med. Phys. 35, 5179-5186] recommending the use of point-dose correction factors. This thesis uses Monte Carlo simulations to demonstrate that the values of these correction factors depend strongly on both detector design and field size, as well as other variables such as detector off-axis position and detector azimuthal angle. Mass density is found to be the principal determinant of detector water non-equivalence. Furthermore, it is shown that it is possible to compensate for the mass-density of a detector cavity by incorporating additional components of contrasting mass-density into that detector’s design. For small cavities, such design modifications enable the detector’s small- to large- field response ratio to be matched to that of a “point-like” water-structure: ideal detector performance can be achieved across a variety of irradiation conditions. For existing commercial detectors, a Dose Area Product (DAP) formalism is also developed and shown to be much more robust than the point-dose correction factor approach. In conclusion, correction factor values for existing detector designs depend on a host of variables and their calculation typically relies on the use of time-intensive Monte Carlo methods. This thesis indicates that future moves towards density-compensated detector designs or DAP-based protocols can simplify the methodology of small field dosimetry.
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Costa, Nathalia Almeida. "Desenvolvimento e implementação de um objeto simulador para dosimetria de equipamentos Gamma Knife®." Universidade de São Paulo, 2018. http://www.teses.usp.br/teses/disponiveis/85/85131/tde-03072018-095701/.
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A radiocirurgia estereotáxica é um procedimento que trata principalmente lesões intracranianas para destruir células tumorais inacessíveis cirurgicamente. O Gamma Knife® é uma unidade de radiocirurgia estereotáxica que trata lesões exclusivamente cerebrais com feixes de 60Co de forma não-invasiva. De forma a garantir a acurácia na entrega da dose, esse tipo de equipamento necessita de detectores adequados para determinar a dose a ser entregue com exatidão. O protocolo da IAEA, TRS 483, é um guia padronizado para procedimentos dosimétricos e indicação de detectores relativos à dosimetria de referência de campos pequenos usados em feixes de radioterapia. Este trabalho seguiu as recomendações do TRS 483 na dosimetria do Gamma Knife® e foram realizadas medições utilizando duas câmaras de ionização, Exradin A16 e PTW Pinpoint 3D 31016, a alanina como detector de referência e os objetos simuladores indicados pela Elekta, fabricante do Gamma Knife®, sendo um de ABS e outro de Solid Water®. Um objeto simulador de acrílico foi construído, com as mesmas dimensões dos indicados pela Elekta, e implementado na dosimetria de equipamentos Gamma Knife®. A calibração das câmaras de ionização utilizadas foi realizada em um laboratório padrão primário e o coeficiente de calibração obtido foi utilizado no cálculo da dose absorvida na água dessas câmaras. Os estudos e testes dosimétricos realizados com o objeto simulador construído demonstraram resultados próximos aos simuladores existentes. Todos os fatores de correção indicados pelo TRS 483 foram considerados para o cálculo da dose absorvida. Foi constatado que o novo objeto simulador pode ser utilizado na dosimetria de Gamma Knife® e também para calibração de câmaras de ionização de volume pequeno, garantindo uma configuração de dosimetria e calibração similares e proporcionando ao usuário uma calibração próxima à configuração da dosimetria clínica. O projeto e a execução do objeto simulador utilizado neste trabalho visa contribuir para o TRS 483, de forma que seja utilizado na aplicação da dosimetria de Gamma Knife® e na calibração de câmaras de ionização de volume pequeno.Stereotactic radiosurgery is a procedure that primarily treats intracranial lesions to destroy tumor cells that are inaccessible surgically. Gamma Knife® is a stereotactic radiosurgery unit that treats exclusively cerebral lesions using 60Co beams non-invasively. In order to ensure accurate dose delivery, this type of equipment requires adequate detectors to determine the dose is delivered accurately. The IAEA TRS 483 protocol is a standardized guide for dosimetric procedures and indication of detectors for reference dosimetry of small fields used in radiotherapy beams. Following TRS 483 recommendations in Gamma Knife® dosimetry, the measurements performed in this work used two ionization chambers; Exradin A16 and PTW Pinpoint 3D 31016, alanine as reference detector and phantoms indicated by Elekta, Gamma Knife\'s® manufacturer, which are made of ABS and Solid Water®. An acrylic phantom was built with the same dimensions as those indicated by Elekta, and implemented for the Gamma Knife® dosimetry. The calibration of the ionization chambers used was performed in a primary standard laboratory and the calibration coefficient obtained was used to calculate the absorbed dose in the water for these chambers. The dosimetry and tests performed with the acrylic phantom showed results close to the existing phantoms. All the correction factors indicated by TRS 483 were considered for the absorbed dose to water calculation. The new phantom can be used in Gamma Knife® dosimetry and also for calibration of small volume ionization chambers, assuring a similar dosimetry and calibration configuration and providing a calibration close to the clinical dosimetry configuration for the user. The design and implementation of the simulator object used in this work aims to contribute to the TRS 483, for the use in Gamma Knife® dosimetry and in the calibration of small volume ionization chambers.
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Stradiotto, Marco. "A Monte Carlo study to investigate the dosimetric accuracy in the small field regime." Thesis, University of British Columbia, 2007. http://hdl.handle.net/2429/32683.
Full textAbstract:
Stereotactic radiosurgery (SRS) is a technique where an external photon beam is
used to deliver a high dose of ionizing radiation to a small, localized brain lesion. The
treatment fields used in this technique can be of the order of 1 cm for a circular
collimated field. Accurate dosimetry of small-field photon beams such as those used in
stereotactic radiosurgery is difficult because of the presence of steep dose gradients and
lateral electronic disequilibrium (LED). Conventional measurement devices and
treatment planning systems (TPS) have difficulty in determining dose accurately for
small treatment fields where LED is present. Further, the pencil beam algorithm in
certain TPS has been known to have certain limitations in regards to dose calculation in
inhomogeneous media where LED is present. The use of Monte Carlo simulations in
radiotherapy has been established as an accurate way of assessing dose distributions in
inhomogeneous media regardless of LED, field size and for beam obliquity.
In this work a Varian iX linear accelerator equipped with a micro-multileaf
collimator was modeled using the Monte Carlo code BEAMnrc and dose calculations
were performed with the Monte Carlo code DOSXYZnrc to accurately predict relative
dose factors (RDF) and inhomogeneity correction factors (ICF) in inhomogeneous bone
and air phantoms. The results were compared with ion chamber measurements and
calculations with the TPS BrainSCAN (Brainlab, Germany). After benchmarking our
BEAMnrc linear accelerator with measured data and obtaining good agreement, further
calculations to determine RDF and ICF were performed with DOSXYZnrc. The RDF
values obtained with Monte Carlo simulation agree to within 1% with their corresponding
measured values and the ICF values obtained with Monte Carlo simulation agree to
within 1% with their corresponding values obtained from BrainSCAN.
Based on our observations and data we conclude that Monte Carlo is an effective
tool for calculating RDFs for small radiation fields where LED is a problem and for
quantifying the possible limitations of a dose calculation algorithm in the region of LED.
In general, we also conclude that the TPS BrainSCAN inaccurately predicts dose in the
presence of lower density inhomogeneities and this inaccuracy becomes worse for
smaller field sizes.Science, Faculty of
Physics and Astronomy, Department of
Graduate
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Benmakhlouf, Hamza. "Key Data for the Reference and Relative Dosimetry of Radiotherapy and Diagnostic and Interventional Radiology Beams." Doctoral thesis, Stockholms universitet, Fysikum, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-114413.
Full textAbstract:
Accurate dosimetry is a fundamental requirement for the safe and efficient use of radiation in medical applications. International Codes of Practice, such as IAEA TRS-398 (2000) for radiotherapy beams and IAEA TRS-457 (2007) for diagnostic radiology beams, provide the necessary formulation for reference and relative dosimetry and the data required for their implementation. Research in recent years has highlighted the shortage of such data for radiotherapy small photon beams and for surface dose estimations in diagnostic and interventional radiology, leading to significant dosimetric errors that in some instances have jeopardized patient’s safety and treatment efficiency. The aim of this thesis is to investigate and determine key data for the reference and relative dosimetry of radiotherapy and radiodiagnostics beams. For that purpose the Monte Carlo system PENELOPE has been used to simulate the transport of radiation in different media and a number of experimental determinations have also been made. A review of the key data for radiotherapy beams published after the release of IAEA TRS-398 was conducted, and in some cases the considerable differences found were questioned under the criterion of data consistency throughout the dosimetry chain (from standards laboratories to the user). A modified concept of output factor, defined in a new international formalism for the dosimetry of small photon beams, requires corrections to dosimeter readings for the dose determination in small beams used clinically. In this work, output correction factors were determined, for Varian Clinac 6 MV photon beams and Leksell Gamma Knife Perfexion 60Co gamma-ray beams, for a large number of small field detectors, including air and liquid ionization chambers, shielded and unshielded silicon diodes and diamond detectors, all of which were simulated by Monte Carlo with great detail. Backscatter factors and ratios of mass energy-absorption coefficients required for surface (skin) determinations in diagnostic and interventional radiology applications were also determined, as well as their extension to account for non-standard phantom thicknesses and materials. A database of these quantities was created for a broad range of monoenergetic photon beams and computer codes developed to convolve the data with clinical spectra, thus enabling the determination of key data for arbitrary beam qualities. Data presented in this thesis has been contributed to the IAEA international dosimetry recommendations for small radiotherapy beams and for diagnostic radiology in paediatric patients.At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 6: Manuscript.
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Nobecu, Lazola Jethro. "Cobalt teletherapy small field dosimetry." Thesis, 2017. http://hdl.handle.net/10539/23465.
Full textAbstract:
A research report submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree of Master of Science in the field of Medical Physics.
2017Aim The aim of this research was to contribute to clinical implementation of the small field dosimetry Code of Practice (CoP) that is due to be published by the International Atomic Energy Agency (IAEA) in collaboration with the American Association of Physicists in Medicine (AAPM) (1). A 6 cm × 6 cm virtual machine-specific reference (fmsr) field was established in a clinical 60Cobalt teletherapy beam used for conventional radiotherapy at Charlotte Maxeke Johannesburg Academic Hospital, and relative output factors were measured down to a set field size of 1 cm × 1 cm using three different models of Physikalisch-Technische Werkstatten (PTW) small field ionization chambers. Materials and Methodology The measurements were all performed on a Cobalt teletherapy unit (MDS Nordion Equinox, S/N 2009) in a PTW MP3 water phantom. The small field ionization chambers that were used were a PTW 31016 3D pinpoint 0.016 cm3, a PTW 31006 pinpoint 0.015 cm3 and a PTW 31010 semiflex 0.125 cm3. A calibrated PTW 30013 Farmer 0.6 cm3 ionization chamber was used to provide traceability for the cross calibration. A ―daisy chain‖ methodology was used to perform the cross calibration in a virtual fmsr field of 6 cm × 6 cm and then establish the absolute dose rate in a 4 cm × 4 cm field. Relative output factors as a function of field size were measured with each small field ionization chamber and then compared to published results. Results Small square fields from a 60Cobalt beam were created using the secondary collimators integrated into the unit. Equivalent square fields were calculated using the profiles obtained by the three ionization chambers during scanning and were in agreement with the ones that were programmed into the console. The coincidence of the central axis of the beam and the point of measurement for each detector was determined from the beam profiles. The cross calibration and daisy chain measurements resulted in a consistent dose rate of within + 0.6% in the 4 cm × 4 cm field when measured with the four different ionization chambers. For 6 cm × 6 cm, 4 cm × 4 cm, 3 cm × 3 cm and 2 cm × 2 cm field sizes, relative output factors obtained from the uncorrected detectors’ response agreed to within + 0.8 % between the three small field ionization chambers. The variation in the 1 cm × 1 cm field size was + 8.1 %. When compared to published data, large differences in field size correction factors were obtained. Conclusion Small field dosimetry in a 60Cobalt photon beam using three different PTW small field ionization chamber models was investigated. A cross calibration in a virtual msr field was done followed by a daisy chain process to determine the dose rate in a small field. Dose profiles and relative output factors were then measured and compared. The lack of lateral charge particle equilibrium and volume averaging effect was evident when using the PTW 31010 semiflex chamber in a 1 cm × 1 cm field. The PTW 31006 pinpoint and 31016 3D pinpoint were in close agreement for field sizes down to 1 cm × 1 cm with the 3D pinpoint performing as the best detector in this study. The optimal positioning of a detector should be determined from beam profile scans and not the engineering diagrams. The PTW 31016 3D pinpoint and PTW 31006 pinpoint are recommended for the determination of output factors in small field sizes. However, field output correction factors are required for both detectors in field sizes under 2 cm × 2 cm. Small field data published in the British Journal of Radiology (BJR) Supplement 25 (2) should not be used to benchmark dosimetry in modern 60Cobalt teletherapy units.
MT 2017
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M'ule, Barbara Chanda. "Small field dosimetry of high-energy electron beams." Thesis, 2009. http://hdl.handle.net/10539/6817.
Full textAbstract:
ABSTRACT
Purpose: A comparative dosimetric study was conducted with four detectors manufactured
by PTW-Freiburg (Germany) (Markus – type 23343, Advanced Markus – type 34045,
PinPoint – type 31006 and a diode type – 60012) for small field high-energy electron beams.
The dosimetry of Intraoperative Radiation Therapy (IORT) is challenging because of the type
of tertiary collimation used. The cones increase low energy electron contamination and
detectors with low energy dependence and high spatial resolution should be used.
Materials and Methods: Dosimetric measurements were made using all four detectors.
Two applicator systems were studied: The small-field system has 9 cylindrical straight end
cones of inner diameters 2 to 8 cm increasing in steps of 1 cm, and two bevel end cones of 2
and 3 cm with a bevel angle of 45°. The periscopic electron cone system is provided with
three set of cylindrical cones of inner diameter 3.2, 3.8 and 4.5 cm with straight and bevel
ends with bevel angle 30°.
Results: All dosimetric data was compared to the Markus chamber. The percent error for the
absolute dose measurements for the Advanced Markus and the PinPoint were found to be
3.2% and 5.1% respectively. For the small-field cone system the percent difference in output
factors (OF) for the Markus chamber and all other detectors was found to be less than 2.6%
for the straight cones and less than 4.8% for the bevelled cones. The percent difference for
the Markus and all other detectors was found to be less than 1.9% for straight cones and 4.2%
for the bevelled cones for the periscopic system.
Conclusions: The Markus, Advanced Markus and the diode detector, may be used for the
relative dosimetry of small electron fields. The Markus and diode detector should be crosscalibrated
for absolute dose measurements. The PinPoint chamber should not be used for the
absolute dosimetry of small electron fields, unless the quality correction factor is
determined for the beam quality used. A comparative study should be made with the
PinPoint oriented in the two possible positions to determine which position is more suitable
for electron dosimetry. Spencer-Attix stopping power ratios for water to air ( ) must be
calculated using Monte Carlo simulations for the two applicator systems used.
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Ko, Hui-Ling, and 柯慧玲. "Dosimetry of Small and Elongated Field Electron Beams for Preventing Arteriovenous Fistula Stenosis." Thesis, 2002. http://ndltd.ncl.edu.tw/handle/23504633157415471977.
Full textAbstract:
碩士國立陽明大學
放射醫學科學研究所
90
In recent years, more and more end-stage renal disease patients are noted. In 2000, over 300,000 people were being treated for end-stage renal disease in the United States. Hemodialysis is used in over 80% of all patients who undergo dialysis. When suitable anatomy exists, primary arteriovenous fistula remains the procedure of choice for hemodialysis access. Unfortunately, almost 50% of patients who have an arteriovenous grafts placed experience graft failure within 1 to 2 years. Distal (venous) anastomotic neointimal hyperplasia is the usual mechanism of graft failure. Because cells that are actively replication are generally radiosensitive, radiation therapy is an attractive approach to local limit smooth muscle cell proliferation, thus decreasing the likelihood of neointimal hyperplasia. Recently, two radiation techniques have been applied to inhibit stenoses of arterio-venous fistula, endovascular irradiation and external irradiation. Advantage of external beam irradiation is a quick dose delivery time. Because no intravascular device is placed, ischemia or thrombosis resulting from vascular damage is less likely. The dialysis accesses (arterio-venous fistula) are uniquely suited for external-beam therapy because of their superficial location, usually just beneath of the skin. However, the area of arterio-venous fistula is small and elongated. The purpose in this study is to evaluate dose distribution of small and elongated field electron beam for further reference in arterio-venous fistula treatment. The percent depth dose, isodose curve, and output factor were measured as a function of electron energies and field size. For electron energies studied, the depth of maximum dose shift towards the surface as the electron beam diameter is reduced by cerrobend shield. In addition, the smaller the diameter of the beam the greater the surface dose, and the dose fall-off region becomes more gradual. If the mean depth of arterio-venous fistula is 1.5 cm and 5mm in width, the safe margin should use 8mm in long and short axis to cover treatment target adequately. Besides, the smaller distance between blocks and phantoms, the better dose distribution. The study supports a vital treatment dose system of small and elongated field size electron beam for not only improved arterio-venous fistula patency, but also provided accurate dose delivery of skin, head and neck tumor treatment.
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Johnstone, Christopher Daniel. "Microcomputed tomography dosimetry and image quality in preclinical image-guided radiation therapy." Thesis, 2019. http://hdl.handle.net/1828/10789.
Full textAbstract:
Motivated by the need to standardize preclinical imaging for image-guided radiation therapy (IGRT), we examine the parameters that influence microcomputed tomography (microCT) scans in the realm of image quality and absorbed dose to tissue, including therapy beam measurements of small fields.
Preclinical radiation research aims to understand radiation-induced effects in living
tissues to improve quality of life. Small targets and low kilovoltage x-rays create challenges that do not arise in clinical radiation therapy.
Evidence based on our multi-institutional study reveals a considerable aberration
in microCT image quality from one institution to the next. We propose the adoption
of recommended tolerance levels to provide a baseline for producing satisfactory and
reproducible microCT image quality scans for accurate dose delivery in preclinical
IGRT.
Absorbed dose imparted by these microCT images may produce deterministic effects that can negatively influence a radiobiological study. Through Monte Carlo (MC) methods we establish absorbed microCT imaging dose to a variety of tissues and murine sizes for a comprehensive combination of imaging parameters. Radiation beam quality in the small confines of a preclinical irradiator is also established to quantify the effects of beam scatter on half-value layer measurements.
MicroCT scans of varying imaging protocols are also compared for murine subjects.
Absorbed imaging dose to tissues are established and presented alongside their respective microCT images, providing a visual bridge to systematically link image quality and imaging dose.
We then characterize a novel small plastic scintillating dosimeter to experimentally measure microCT imaging and therapy beams in real-time. The presented scintillating
dosimeter is specifically characterized for the low energies and small fields found in
preclinical research. Beam output is measured for small fields previously only achievable
using film. Finally, quality assurance tests are recommended for a preclinical IGRT unit.
Within this dissertation, a narrative is presented for guiding preclinical radiotherapy
towards producing high quality microCT images with an understanding of the absorbed imaging dose deposited to tissues, including providing a tool to measure small radiation fields.Graduate
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Alhakeem, Eyad Ali. "Dosimetry at extreme non-charged particle equilibrium conditions using Monte Carlo and specialized dosimeters." Thesis, 2018. https://dspace.library.uvic.ca//handle/1828/10123.
Full textAbstract:
Radiotherapy is used in clinics to treat cancer with highly energetic ionizing particles. The radiation dose can be measured indirectly by means of radiation detectors or dosimeters. The dose deposited in a detector can be related to dose deposited in a point within the patient. In theory, however, this is only possible under charged particle equilibrium (CPE). The motivation behind the dissertation was driven by the difficult, yet crucial, dosimetry in non-CPE regions. Inaccurate dose assessment performed with standard dosimetry using ionization chambers may significantly impact the outcomes of radiotherapy treatments. Therefore, advanced dosimetry methods tailored specifically to suit non-CPE conditions must be used. This work aims to improve dosimetry in two types of non-CPE conditions that pose dosimetric challenges: regions near interfaces of tissues with low- and high- density media and in small photon fields.
To achieve the main dissertation objectives, an enhanced film dosimetry protocol with a novel film calibration approach was implemented. This calibration method is based on the percent depth dose (PDD) tables and was shown to be efficient and accurate. As a result, the PDD calibration method was used for the film dosimetry process throughout the dissertation work.
Monte Carlo (MC) calculations for the small field dosimetry were performed using phase-space files (PSFs) provided by Varian for TrueBeam linac. The MC statistical uncertainty in these types of calculations is limited by the number of particles (due to latent variance) in the used PSFs. This study investigated the behaviour of the latent variances (LV) with beam energy, depth in phantom, and calculation resolution (voxel size). LV was evaluated for standard 10x10 cm2 fields as well as small fields (down to 1.3 mm diameter). The results showed that in order to achieve sub-percent LV in open 10x10 cm2 field MC simulations a single PSF can be used, whereas for small SRS fields (1.3—10 mm) more PSFs (66—8 PSFs) would have to be summed.
The first study in this dissertation compared the performance of several dosimetric methods in three multi-layer heterogeneous phantoms with water/air, water/lung, and water/steel interfaces irradiated with 6 and 18 MV photon beams. MC calculations were used, along with Acuros XB, anisotropic analytical algorithm (AAA), GafChromic EBT2 film, and MOSkin dosimeters. PDDs were calculated and measured in these heterogeneous phantoms. The result of this study showed that Acuros XB, AAA, and MC calculations were within 1% in the regions with CPE. At media interfaces and buildup regions, differences between Acuros XB and MC were in the range of +4.4% to -12.8%. MOSkin and EBT2 measurements agreed to MC calculations within ~ 2.5%-4.5%. AAA did not predict the backscatter dose from the high-density heterogeneity. For the third, multilayer lung phantom, 6 MV beam PDDs calculated by all treatment planning system (TPS) algorithms were within 2% of MC. 18 MV PDDs calculated by Acuros XB and AAA differed from MC by up to 3.2 and 6.8%, respectively. MOSkin and EBT2 each differed from MC by up to 3%. All dosimetric techniques, except AAA, agreed within 3% in the regions with particle equilibrium. Differences between the dosimetric techniques were larger for the 18 MV than the 6 MV beam. This study provided a comparative performance evaluation of several advanced dosimeters in heterogeneous phantoms. This combination of experimental and calculation dosimetry techniques was used for the first time to evaluate the dose near these interfaces.
The second study in the dissertation aims to improve dose measurement accuracy in small radiotherapy fields. Field output factors of 6 MV beams from TrueBeam linear accelerator (linac) collimated with 1.27-40 mm diameter cones were calculated and measured using MC and EBT3 films. A set of detector specific correction factors for two widely used dosimeters (EFD-3G diode and PTW-60019 microDiamond detectors) were determined based on GafChromic EBT3 film measurements and calculated using MC methods. MC calculations were performed for microDiamond detector in parallel and perpendicular orientations relative to the beam axis. The result of this study showed that the measured OFs agreed within 2.4% for fields ≥10 mm. For the cones of 1.27, 2.46, and 3.77 mm diameter maximum differences were 17.9%, 1.8% and 9.0%, respectively. MC calculated OF in water agreed with those obtained using EBT3 film within 2.2% for all fields. MC calculated output correction factors for microDiamond detector in fields ≥10 mm ranged within 0.975-1.020 for perpendicular and parallel orientations. MicroDiamond detector correction factors calculated for the 1.27, 2.46 and 3.77 mm fields were 1.974, 1.139 and 0.982 with detector in parallel orientation, and these factors were 1.150, 0.925 and 0.914 in perpendicular orientation. EBT3 and MC obtained correction factors agreed within 3.7% for fields of ≥3.77 mm and within 5.9% for smaller cones. This work provided output correction factors for microDiamond and EFD-3G detectors in very small fields of 1.27 – 3.77 mm diameter and demonstrated over and under-response of these detectors in such fields. These correction factors allow improve the accuracy of dose measurements in small photon fields using these detectors.Graduate
2019-08-30
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Chu, Che-Hsing, and 朱哲興. "Dosimetry for small photon fields using normoxic polymer gel dosimeter." Thesis, 2007. http://ndltd.ncl.edu.tw/handle/04963426953854692441.
Full text
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Msimang, Zakithi Lungile Mpumelelo. "Accuracy and associated measurement uncertainties in clinical dosimetry data for static small fields." Thesis, 2020. https://hdl.handle.net/10539/31402.
Full textAbstract:
A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy, 2020Developments in radiotherapy techniques and technologies have contributed to an increase in the use of small fields. Small fields are used in stereotactic treatments and large uniform or non-uniform fields that are composed of small fields such as for intensity modulated radiation therapy (IMRT). Implementation has predated guidance documents for clinical dosimetry. The first international Code of Practice (CoP) for small field dosimetry was only published in 2017 by the International Atomic Energy Agency (IAEA), in collaboration with the American Association of Physicists in Medicine (AAPM). There is a lack of data quantifying the accuracies linked with the use of small fields. Estimating and publishing uncertainties for measurement capabilities is standard practice for primary and secondary standard laboratories that operate within a rigorous total quality management system. This is not necessarily the case with clinical dosimetry measurements performed at hospitals, where there is alack of published uncertainties for each of the steps used in the determination of the field output actors (FOF) for small fields. In this study, the accuracy of detector positioning in small field clinical dosimetry measurements were evaluated in 6 MV and cobalt teletherapy beams with different collimation systems. In addition, the impact of two different methods of calculating the equivalent square, constancy and reproducibility of field output factors (FOFs) for different detectors, and machine stability over time, was evaluated. The influence of a reference detector was investigated. The uncertainties of all measurements were determined. For the linear accelerator data, the integrated multileaf collimator (MLC) and jaw were used as well as demountable stereotactic circular cones. The data from the study highlighted the importance of verifying Central Axis (CAX) for independent measurement set ups and not relying on a visual set up using the field projection, the manufacturer’s specifications of a detector or an automated water tank positioning system. A variation in CAX of 0.8 mm for in plane and 1.6 mm for cross plane was found to yield a 32 % variation in the FOF for Sclin of 0.6 cm. The solid-state detectors used in the study performed better than the air ionisation detectors and are thus recommended as detectors of choice. The study proved that the need for and frequency of the MLC calibration greatly affects the FOF, and lack of MLC maintenance will result in a gradual, unpredictable change in Sclin. A 3-monthly calibration period of the machine used in this study yielded results that were within the measurement uncertainties for the determination of Sclin, and it was concluded that this frequency was sufficient to achieve the required outcomes. Data were compared to measurements provided by other hospitals in South Africa, standard data sets (BJR25) and other international hospitals that participated in the IAEA coordinated research project [E24021: “Testing of Code of Practice on Small Field Dosimetry”].Comparison of measured data to that published in the British Journal of Radiology (BJR) Supplement No. 25 of 1996 showed that BJR 25 data for 6MV and cobalt teletherapy should not be adopted for Sclin ≤ than 6 cm. Local data should be determined experimentally. For reference measurements, the standard uncertainty contributed by the traceability process of daisy-chaining, contributed the most significant uncertainty. For relative dosimetry measurements, the standard uncertainty associated with the determination of the FOF contributed the most significant uncertainty. These were identified as the two high risk areas in the dosimetry chain for small static field dosimetry. As such, dosimetry audits for small fields should focus on the FOF and reference dose determination in field sizes ≤ 2 cm
CK2021
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Haider, Jacob Abraham. "Dosimetry studies of small fields in homogeneous and inhomogeneous media for high energy photons." Thesis, 1995. http://hdl.handle.net/2429/7370.
Full textAbstract:
Dose decreases rapidly for photon field sizes smaller than the range of the laterally
scattered electrons. The reduction in dose leads to dose non-uniformity and the degree of
dose non-uniformity depends on the shape of the tumor. The dose reduction due to lateral
electronic nonequilibrium increases with increasing photon energy. Small tumors are best
treated with lower energy photons.
We modified the primary and scatter dose model to include the effect of lateral
electronic nonequilibrium. The dose model was verified experimentally for various
geometries and is in good agreement with the measurements.
We developed a new cavity theory which includes secondary electron
backscattering from the medium into the cavity. The proposed theory gives better
agreement with experiments in aluminium, copper and lead for Co-60 ƴ-rays and 10 MV
x-rays than do the Burlin and Kearsley cavity theories. A method for obtaining an
ionization chamber correction factor for measuring dose in inhomogeneous media is also
given.
There is a significant dose reduction in lung as compared to normal density tissue
for small fields. The dose reduction in lung increases with decreasing field size and
increasing photon energy. Results of the measurements suggest that a tumor in tissue
surrounding lung would have better dose uniformity if the direction of the photon beam
is such that the tumor resides in the proximal side of the tumor-lung interface. Tumors
in lung and surrounding the lung have better dose uniformity if treated with lower energy
photons.
Significant dose reduction was also observed near the air-tissue interface. The dose
perturbation increases with increasing air-cavity thickness, decreasing field size and
increasing photon energy. Results of the measurements again suggest that a tumor in
tissue surrounding air-cavities, such as the bronchial tube, would have better dose
uniformity if the direction of the photon beam is such that the tumor resides in the
proximal side of the tumor-air interface. And again, lower energy photons provide better
dose uniformity for tumors surrounding the air-cavity.
The presence of bone in tissue causes only modest dose perturbation for photon
energies between 2 MV and 20 MV x-rays.
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Hsin-fa, Liao, and 廖信發. "The study of the miniature plastic scintillating fiber detector ─ A dosimeter for small field radiotherapy application." Thesis, 2002. http://ndltd.ncl.edu.tw/handle/67004676340468207861.
Full textAbstract:
碩士國立清華大學
原子科學系
90
The purpose of this study is trying to develop a miniature plastic scintillation fiber detecting system used for radiation measurement. The fluorescence light released promptly from the scintillator under the ionizing radiation exposed, therefore, the dose rate can be evaluated by its intensity of the light emitted. The work of the developed detecting system consists mainly (1) to develop a high impendence of high amplification current amplifier, which could convert the very low current signal into readable voltage output, and (2) to construct the detector of opto-electric device, which the Bicron type BCF-20 plastic scintillating fiber of 1 mm × 5 mm L was used as the detection probe to integrate with a 10 m long plastic optic fiber as the light transmission medium, and the opto-electric converting device was either by a photomultiplier tube or a photodiode. The experiments divided into two groups. One has photomultiplier-coupled of optic fiber system and the other has photodiode-coupled one. Of the group content two different type probe, one was clad with thin sheet of pure gold and the other was bared. The results show the detection sensitivity were no significantly difference between the gold clad and bared one. The photomultiplier-coupled device is suitable to determine the dose rate below 350 cGy/min, and the photodiode-coupled one is suitable only for the dose rate above 5 Gy/min. In small field cone beam experiments, which carried out of the dose profile measurements of different depth in water phantom. Our developed system shows much better performance than the conventional ion chamber, especially in the region of penumbra not only has excellent spatial resolution but also has accurate of dose gradient.
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Αναστάσης, Βασιλάκης. "Δοσιμετρία μικρών πεδίων." Thesis, 2013. http://hdl.handle.net/10889/7787.
Full textAbstract:
Στόχος τη ακτινοθεραπείας είναι η χορήγηση της θεραπευτικής δόσης με τη
μέγιστη δυνατή ακρίβεια. Αυτό συνεπάγεται τον σωστό καθορισμό της
ακτινοβολούμενης περιοχής καθώς και την ακριβή εναπόθεση της δόσης. Αυτή η
διπλωματική εργασία ασχολείται με την προσπάθεια για ακριβή υπολογισμό και
εναπόθεση της δόσης για πεδία ακτινοβόλησης τα όποια είναι μικρότερα από 5x5cm.
Όταν το μέγεθος του πεδίου μικρύνει τότε η μέτρηση και ο υπολογισμός της δόσης με
κλασικές μεθόδους δε είναι πλέον ακριβείς καθώς παράγοντες όπως η πλευρική
ηλεκτρονική ισορροπία, το μέγεθος και είδος του ανιχνευτή καθώς και το μέγεθος της
πηγής που είναι ορατό από κάθε σημείο, πρέπει να ληφθούν υπόψη. Στη παρούσα
εργασία χρησιμοποιήθηκε το λογισμικό Mephysto για να μετρήσουμε την πραγματική
δόση που δίνει ο γραμμικός επιταχυντής της εταιρείας ELEKTA σε δέσμες φωτονίων
ενέργειας 6 ΜV χρησιμοποιώντας έναν ανιχνευτή Pin Point της εταιρείας PTW. Στη
συνέχεια συγκρίθηκαν αυτά τα αποτελέσματα (προφίλ δόσης, κατά βάθος δόση PDD)
με τα αποτελέσματα που δοθήκαν από το υπολογιστικό σύστημα σχεδιασμού θεραπειών
(Treatment Planning System) Oncentra Master Plan της εταιρείας Nucletron.
Παρατηρήθηκε απόκλιση μεταξύ αυτών των δυο μεθόδων ελαφρώς μικρότερη του 3%.
Αυτή η απόκλιση οφείλεται στο ότι ο εικονικός γραμμικός επιταχυντής που έχει
δημιουργηθεί στο σύστημα Oncentra Master Plan για τον υπολογισμό της δόσης,
δημιουργήθηκε ώστε να αποδίδει πλησιέστερα αποτελέσματα σε αυτά του ELEKTA για
πεδία ακτινοβόλησης που έχουν μεγάλη κλινική χρήση (5 έως 15 cm αν διάσταση).
Όταν όμως τα πεδία μικρύνουν (κάτω από 5cm αν διάσταση) τότε έχουμε απόκλιση από
της πραγματικές τιμές. Αλλάζοντας το φαινομενικό μέγεθος της πηγής στο Oncentra
Master Plan καταφέραμε να φέρουμε τους υπολογισμούς από το Oncentra Master Plan
πάρα πολύ κοντά στις μετρήσεις του Mephysto. Δημιουργήθηκε έτσι ένα νέο εικονικό
μηχάνημα στη βάση δεδομένων του Oncentra Master Plan με το όνομα Sli Patras SRS,
οι παράμετροι του οποίου (φαινομενικό μέγεθος πηγής) έχουν βελτιστοποιηθεί για
ακριβέστερους υπολογισμούς δόσης για μικρά πεδία. Με αυτό το μηχάνημα
καταφέρθηκε ακριβέστερος υπολογισμός της δόσης, με αποκλίσεις μικρότερες από
1.5%, για μικρά πεδία, σε σύγκριση με το προηγούμενο μηχάνημα Sli Patra. Το νέο αυτό
μηχάνημα επιτρέπει ακριβέστερους υπολογισμούς για μικρά πεδία και έχει πλέον
υιοθετηθεί και χρησιμοποιείται στην κλινική ρουτίνα στο Π.Γ.Ν. Πατρών.Small field dosimetry in sterotactic cancer radiation therapy.
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Δροσάτου, Καλλιόπη. "Διασφάλιση ποιότητας στη στερεοτακτική ακτινοθεραπεία και δοσιμετρία μικρών πεδίων." Thesis, 2014. http://hdl.handle.net/10889/8361.
Full textAbstract:
Η στερεοτακτική μέθοδος ακτινοθεραπείας και ακτινοχειρουργικής είναι μια νεότερη μέθοδος της ογκολογίας, που αποδεδειγμένα υπερέχει έναντι της συμβατικής μεθόδου ακτινοβόλησης, ιδιαίτερα όταν συνδυάζεται με τελευταίες τεχνικές ακτινοβόλησης, όπως το IMRT, ArcTherapy and VMAT. Χαρακτηρίζεται δε από ακτινοβόληση με ιδιαίτερα υψηλές δόσεις, συνήθως πολύ μικρών όγκων, της τάξεως του εκατοστού. Εξαιτίας αυτών, είναι επιτακτική η ανάγκη για μέγιστη ακρίβεια και αποφυγή λαθών, καθώς δεν υπάρχουν περιθώρια σφάλματος!
Κάθε ακτινοθεραπευτικό κέντρο οφείλει λοιπόν να ακολουθεί ένα ολοκληρωμένο και ιδιαίτερα αυστηρό πρόγραμμα ποιοτικού ελέγχου, θεσπίζοντας μια σειρά ελέγχων σε ημερήσια, εβδομαδιαία, μηνιαία και ετήσια βάση.
Μέθοδος: Επειδή δεν υπάρχει ένα εντεταλμένο πρωτόκολλο Ποιοτικού Ελέγχου Στερεοτακτικής Ακτινοθεραπείας με Γραμμικό Επιταχυντή, αρχικά στην εργασία αυτή έγινε μια βιβλιογραφική μελέτη για να βρεθεί το state of the art αυτού του ζητήματος και να εντοπιστούν, σε πρώτη φάση, οι επιμέρους ποιοτικοί έλεγχοι που προτείνονται για τα διάφορα μέρη της στερεοτακτικής ακτινοβόλησης (εξοπλισμός και διαδικασία). Κατόπιν, προσδιορίστηκε μια συνολική, βέλτιστη και κατάλληλη για την πλειοψηφία των ακτινοθεραπευτικών κέντρων, λίστα ελέγχων για τη Διασφάλιση της Ποιότητας στην Στερεοτακτική Ακτινοθεραπεία.
Αποτελέσματα: Ο Πίνακας των Ελέγχων που προέκυψε είναι – ως όφειλε – σύμφωνος με τα επιμέρους διεθνή πρωτόκολλα της Ευρώπης και Αμερικής. Βάσει αυτού προτείνεται ένα ολοκληρωμένο Πρόγραμμα Διασφάλισης Ποιότητας για Ακτινοθεραπευτικά Κέντρα που εφαρμόζουν στερεοταξία, το οποίο μπορεί να διαμορφωθεί από κάθε ακτινοθεραπευτικό κέντρο, βάσει των ιδιαιτεροτήτων αυτού, αλλά και να προσαρμοστεί σε μελλοντικές τεχνολογικές αλλαγές. Η εφαρμογή τέτοιων ελέγχων, τέλος, διερευνήθηκε στο ιδιωτικό θεραπευτήριο ΜΕΤΡΟΠΟΛΙΤΑΝ του Φαλήρου.
Συμπεράσματα: Το πρόγραμμα ελέγχων που προτείνεται, αν και είναι σύμφωνο με τα επιμέρους διεθνή πρωτόκολλα ποιοτικού ελέγχου για τον εξοπλισμό και τη διαδικασία της ακτινοθεραπείας, πρέπει περαιτέρω να διερευνηθεί επί της κλινικής πράξης από ακτινοθεραπευτικά κέντρα, προκειμένου να βρεθούν τυχόν αδυναμίες και ελλείψεις και τελικώς να αποτελέσει τη βάση ενός μελλοντικού Πρωτόκολλου Διασφάλισης Ποιότητας Στερεοτακτικής Ακτινοθεραπείας με Γραμμικό Επιταχυντή, εγκεκριμένου από τους αρμόδιους διεθνείς φορείς και οργανισμούς.Stereotactic Radiation Therapy (SRT) and Stereotactic RadioSurgery (SRS) are new advanced oncologic treatment modalities, which proved superior to the conventional method of irradiation, particularly when combined with latest irradiation techniques, such as IMRT, ArcTherapy and VMAT. They apply very high doses, to – usually – very small volumes (centimeters range). These characteristics mean great need for maximum accuracy and avoid mistakes, as there is no room for error! Therefore, every radiotherapy center must follow a very strict and comprehensive quality control program, adopting a series of checks on a daily, weekly, monthly and yearly basis. Method: Because there is no specific Quality Control Protocol for Stereotactic Radiotherapy using Linear Accelerator, the first part of this work was a literature study to find the state of the art of this issue and find, at first, the proposed quality controls for each part of stereotactic irradiation (equipment and process). Afterwards an overall checklist for Quality Assurance in Stereotactic Radiotherapy was defined, which was assessed as optimal and suitable for most radiotherapy centers. Results: The resulted table of checks is - as it should - in line with the different international protocols in Europe and America. Based on this, a comprehensive Quality Assurance Program for radiotherapy centers applying stereotaxis, is proposed. This may be configured and modified by each radiotherapy center, according to its specificities, and also adapt to future technological advances. Finally we look into the implementation of such controls, at the METROPOLITAN private hospital of Faliro. Conclusions: Although the recommended Control Program is consistent with the different international quality control protocols for the equipment and process of radiotherapy, the individual radiotherapy centers should further investigate this program when in clinical use, in order to find any deficiency or weakness. The ultimate goal was to create the basis for the future Quality Assurance Protocol in Stereotactic Radiotherapy with Linac, which should be approved by the competent international comities and organizations.