This thesis details an investigation of the suitability of commercially-available single crystal and polycrystalline diamond films made via chemical vapor deposition (CVD) that were not studied previously for use in radiotherapy dosimetry. Novel sandwich-type detectors were designed and constructed to investigate the dosimetric response of diamond films under clinical conditions. Relatively inexpensive diamond films were obtained from three manufacturers: Diamonex, Diamond Materials GmbH and Element Six. Spectrophotometry, Raman spectroscopy and bulk conductivity studies were used to characterize these films and correlate crystalline quality with detector performance. Novel detectors were designed and constructed to investigate detectors under clinical conditions, including Perspex encapsulations and PCBs to minimize fluence perturbations. The dosimetric response of these diamond detectors was examined using a 6 MV beam from a Varian Clinac 600C linear accelerator. Diamond detectors were evaluated by measuring a number of response characteristics. Polycrystalline CVD diamond films from Diamonex (100, 200, 400-μm thicknesses) were considered unsuitable for dosimetric applications due to their lack of stability, low sensitivity, high leakage currents, high priming dose and dependence on dose rate. High-quality polycrystalline diamond films from Diamond Materials (100, 200, 400-μm thicknesses) displayed characteristics that varied with film thickness. A 100-μm film featured slow response dynamics and high priming doses. Thicker films featured suitable dosimetric characteristics, e.g. negligible leakage currents, low priming doses, fast response dynamics and good sensitivity with small sensitive volumes. Element Six single crystal CVD diamond films (500-μm thicknesses) with small sensitive volumes (0.39 mm³) exhibited suitable characteristics for dosimetry. These films showed negligible leakage currents (< 1.25 pA), low priming doses (1–10 Gy), quick response dynamics, high sensitivity (47–230 nC Gy⁻¹) and were weakly dependent on dose rate and directional dependence (±1%). A relatively inexpensive single crystal CVD diamond film from Element Six that exhibited high sensitivity (230 nC Gy⁻¹ at 0.5 V μm⁻¹), amongst other favourable characteristics, was selected for further analyses. An appropriate operating voltage was determined before further clinically relevant measurements could be conducted. This included how changes in an applied electric field affected detector response, and determined whether an optimal operating voltage could be realized within the parameters of conventional instrumentation used in radiation therapy. The results of this study indicated a preference towards using 62.5 V (at ~0.13 V μm⁻¹) out of a range of 30.8–248.0 V for temporal response as required for modulated beams due to its minimal rise time (2 s) and fall time (2 s) yet sufficient sensitivity (37 nC Gy⁻¹) and weak dependence on polarity (±1.5%). Investigations were then performed on the same diamond detector to evaluate its performance under more clinically relevant conditions. Repeatability experiments revealed a temporary loss in sensitivity due to charge detrapping effects following irradiation, which was modelled to make corrections that improved short-term precision. It was shown that this detector could statistically distinguish between dose values separated by a single Monitor Unit, which corresponded to 0.77 cGy. Dose rate dependence was observed when using low, fixed doses in contrast to using stabilized currents and higher doses. Depth dose measurements using this detector compared well with ion chambers and diode dosimeters. Comparisons of initial measurements with values in the literature indicate encouraging results for fields sizes < 4 x 4 cm², but further measurements and comparisons with Monte Carlo calculations are required. Using this detector to make off-axis measurements in the edge-on orientation reduced perturbation of the beam due to its sandwich configuration and thin 150 nm Ag contacts. This diamond detector was found to be suitable for routine dosimetry with conventional radiotherapy instrumentation with a materials cost of < NZ$200.

A dosimetria em radioterapia é importante para garantir a igualdade entre a dose prescrita e a dose entregue ao paciente. Devido às inovações tecnológicas dos aparelhos de radioterapia e das técnicas de tratamento, aumentou-se a dose entregue nas regiões lesionadas. A dosimetria gel polimérica tem se mostrado promissora no controle da qualidade de fontes de radioterapia. O objetivo desse trabalho foi desenvolver um dosímetro gel, MAGIC-fm, para aplicações em dosimetria em Radioterapia e estudando características de sua resposta dosimétrica. O entendimento da construção da resposta dosimétrica, do processo de polimerização e da composição elementar do gel foi realizado através de imagem por ressonância magnética, em intensidades de campo magnético de 3T, 7T e 9,4T, com sequências de leitura de dispersão em T1, Espectroscopia Resolvida no Tempo, Pulse Size Ratio, Relaxometria em T2 e Efeito Overhauser Nuclear. Embora o dosímetro desenvolvido não tenha apresentado dispersão em T1, com essa sequência foi possível determinar R1 e R2, que podem ser utilizados em dosimetria. Através das análises de espectroscopia, verificou-se a contribuição de cada elemento do gel na resposta dosimétrica. O aparecimento de efeitos NOE conforme o aumento da dose, mostrou-se promissora para detecção de dose em gel dosimétrico. O gel MAGIC-fm apresentou sensibilidade 40% maior que a formulação original do gel MAGIC-f, além de saturação de resposta acima de 50 Gy. Utilizando-se a técnica de relaxometria avaliou-se a dependência de resposta com a energia e a taxa de dose em feixes de fótons e elétrons tipicamente utilizados em tratamentos de radioterapia. O processo de calibração do dosímetro foi estudado, concluindo-se que o uso de objetos simuladores com tamanhos similares aqueles de interesse clínico devem ser usados. Uma aplicação de dosimetria em fonte de braquiterapia de alta taxa de dose de 192Ir foi realizada, com resultados comparados com dados do sistema de planejamento. Os resultados apresentaram concordância em mais de 95% dos pontos das regiões analisadas, incluindo regiões de alta dose, concluindo-se que o gel desenvolvido, MAGIC-fm, é promissor no controle da qualidade de fontes de radioterapia / Dosimetry in radiotherapy is important to ensure equality between the prescribed dose and the delivered dose to the patient. Due to the technological innovations of the radiotherapy machines and the treatment techniques, increased the delivered dose in the injured regions. Polymer gel dosimetry has shown promising in the quality control of radiotherapy sources. The purpose of this work was to develop a dosimeter gel, MAGIC-fm, for applications in dosimetry in Radiotherapy and to study characteristics of its dosimetric response. The understanding of dosimetric response construction, the polymerization process and the gel elemental composition was performed by magnetic resonance imaging at magnetic field intensities of 3T, 7T and 9.4T, with reading sequences of T1 dispersion, Time-Resolved Spectroscopy, Pulse Size Ratio, T2 Relaxometry and Nuclear Overhauser Effect. Although the developed dosimeter did not present T1 dispersion, with this sequence it was possible to determine R1 and R2, which can be used in dosimetry. Through the spectroscopy analyzes, the contribution of each gel element in the dosimetric response was verified. The appearance of NOE effects as the dose increased, was shown to be promising for dosimetric gel dose detection. The MAGIC-fm gel showed 40% sensitivity higher than the original MAGIC-f gel formulation, in addition to response saturation above 50 Gy. Using the relaxometry technique, the dependence of energy and dose rate response on photon and electron beams typically used in radiotherapy treatments was evaluated. The dosimeter calibration process was studied, concluding that the use of phantoms with similar sizes of those of clinical interest should be used. An application of dosimetry in a high dose rate 192Ir brachytherapy source was performed, with results compared with data from the planning system. The results showed a concordance in more than 95% of the points of the analyzed regions, including regions of high dose, concluding that the developed gel, MAGIC-fm, is promising in the quality control of radiotherapy sources

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. / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 6: Manuscript.</p>