<p>Radiotherapy is used in many clinics to deliver a sufficient and uniform dose to the cancerous tumours while the dose to normal tissues is minimized. However, there is a possibility of missing the target volume due to patient set up/motion errors, or any fluctuation in treatment delivery. Therefore, accurate dose verification tools are essential to evaluate the delivered dose distribution of the designed treatment plan under realistic treatment conditions.</p> <p>Current research is focused on developing 3D dose verification tools to record the complex dose distributions for quality assurance purposes and the evaluation of new treatment techniques. New and novel materials and read-out techniques suitable for use in hospitals are desirable. The objective of this research is to fabricate a transparent radiochromic gel dosimeter that may be used as quality assurance tool. Also, the fabricated gel must be analyzed using a simple optical read-out technique.</p> <p>Gel dosimeters are gels that undergo some chemical changes upon irradiation as a function of absorbed dose. The absorbed dose may be recorded in three dimensions depending on the type of gel dosimeter. Radiochromic gels are dosimeters that change colour upon irradiation. A radiosensitive dye, leucomalachite green (LMG) is dissolved in a matrix material to record the dose distribution in 3D. LMG changes its colour upon irradiation, and has an absorbance band of 629nm.</p> <p>In this research two different matrix materials were investigated: poly (vinyl alcohol) and gelatin. PVA was studied as the primary agent due to its adjustable mechanical strength and high transparency. PVA has also been studied to have a low diffusion rate when it was used as the matrix material in Fricke gel dosimeters [41]. Even though PVA had all the desired characteristics, fabricating a PVA based radiochromic dosimeter was not successful. Consequently, gelatin was used as the matrix material to fabricate a gelatin-based radiochromic dosimeter.</p> <p>Using gelatin, highly transparent radiosensitive gels were successfully fabricated. The absorbencies of the irradiated gels were measured as a function of absorbed dose, using a 1D set up. After, the gels were formed into 5mm thick films and used as two-dimensional dose verification tools. The relationship between absorbance and absorbed dose for 1D measurement was obtained to be 0.00241± 0.00004 , and 0.0022 ± 0.00007 for 2D gels scaled to a thickness of 1 cm.</p> <p>In all of the experiments the absorbance-dose relationships were similar in slopes, but there was an offset between different batches. The offset was 20% between the different experiments. Moreover, there was less than 5% error associated with the physical set up; the major source of error was due to the production and handling of the mixture, possibly due to the effects of inconsistent heating and UV light exposure.</p> <p>The 2D gels were used to verify the dose distribution for the purpose of quality assurance. Six different complicated beams were delivered to the gels and their dose distributions were compared to their respective Pinnacle Calculated Planar (PCP) dose maps. The difference was found to be about 35% at worst; however, this error may be reduced by utilizing more sophisticated data processing methods. Nevertheless, the images were quite similar above 20Gy. Furthermore, the dose distributions recorded by the gels are qualitatively and quantitatively similar to the (PCP) dose map. Although the fabricated gel dosimeters show some promise as future tools for quality assurance purposes, they must go through many more stages of research to be used clinically.</p> / Master of Science (MSc)

A radioterapia de intensidade modulada (IMRT) é uma das mais modernas técnicas radioterapêuticas que permite a entrega de elevadas e complexas distribuição de doses ao volume tumoral, que necessita de novos métodos para o controle de qualidade dos procedimentos efetuados. Nos serviços de radioterapia costuma-se usar para o controle de qualidade do sistema de planejamento, a câmara de ionização para verificação pontual da dose e um dispositivo com diodos semicondutores (MapCHECK2) para a verificação bidimensional em um plano da fluência planejada, entretanto, para a verificação tridimensional dessas distribuições de doses ainda não há um dosímetro consolidado na rotina clínica. Nesse contexto, para a dosimetria tridimensional se destacam os géis poliméricos. Neste trabalho foram feitas a dosimetria convencional, pontual e bidimensional como se faz na rotina clínica e a dosimetria tridimensional utilizando o gel polimérico Magic-f, que apresenta a distribuição de dose volumétrica. Para este trabalho foi escolhido o tratamento de câncer de próstata, pois na atualidade é um dos tipos de cânceres mais comuns entre os homens. No contexto da dosimetria gel, para se obter a informação volumétrica é necessária uma técnica de imagem, no presente caso foram utilizadas imagens por ressonância magnética (magnetic resonance imaging, MRI). A partir dessas imagens é possível determinar as distribuições de doses processando-as em um software desenvolvido pelo grupo que determina as taxas de relaxação R2 associada à dose absorvida e posteriormente comparar as imagens obtidas com as imagens do sistema de planejamento. Para isso, se obteve dez cortes ao longo de cada simulador físico ou fantom em que sua comparação foi feita com a respectiva fatia do sistema de planejamento, na posição correspondente. Para uma avaliação quantitativa foi utilizado o conceito de índice gama, no critério padrão da radioterapia, 3% da dose e 3mm de distância de concordância. Os resultados obtidos com a dosimetria gel se mostram de acordo com os controles de qualidade convencionais e oferecem uma visão global da distribuição de dose no volume alvo. / The intensity modulated radiotherapy (IMRT) is one of the most modern radiotherapeutic technique that enables the delivery of high and complexes conformational doses to the tumor volume, that requires new methods for the quality assurance of the procedures performed. Radiotherapy services usually perform quality assurance of the planning system with the ionization chamber for spot-checking and an array of semiconductor diodes (MapCHECK2) to check on a two-dimensional plane, however for tridimensional dose verification does not exist an established dosimeter in the clinical routine. In this context, for three-dimensional dosimetry the polymeric gels were used. In This work the conventional one and two-dimensional dosimetry as employed in the clinical routine, and the three-dimensional dosimetry using polymer gel MAGIC- f, which provide the volumetric dose distribution. Prostate cancer clinical cases were chosen for this work because this kind of tumor is one of the most common cases in male individuals. In the context of dosimetry gel to obtain volumetric information an imaging technique is necessary, in this case the magnetic resonance imaging (MRI), was used to measure the dose. From these images it is possible to determine the distributions of doses processing them in a software developed by our research group that determines R2 relaxation rates associated with the absorbed dose and subsequently compare the images obtained with the images of the planning system. For this, ten slices were obtained along each phantom, and comparisons were made with the respective slice of the treatment planning system, in the corresponding position. For a quantitative evaluation of the gamma index , in the standard criterion in radiotherapy, 3 % dose and 3 mm distance to agreement was used. The results obtained shown that gel dosimetry agrees with the conventional quality controls and provide an overview of dose distribution in the target volume.

The current state of a prototype fan-beam optical computed tomography scanner for three-dimensional radiation dosimetry has been presented. The system uses a helium-neon laser and a line-generating lens for fan-beam creation. Five photodiode arrays form an approximate arc detector array of 320-elements. Two options of physical collimators provide two levels of scatter-rejection: single-slot (SS) and multi-hole (MH). A pair of linear polarizers has been introduced as a means of light intensity modulation. This work examined: (i) the characterization of system components, (ii) data acquisition & imaging protocols, and (iii) the scanning of an nPAG dosimeter. (i): The polarizer-pair method of light intensity modulation has been calibrated and the polarization sensitivity of the detector array was evaluated. The relationship between detected values and both light intensity and photodiode integration time was examined. This examination indicated the need for an offset correction to treat all data acquired by the system. Data corruption near the edges of each photodiode array was found to cause ring artefacts in image reconstructions. Two methods of extending the dynamic range of the system---via integration time and light intensity---were presented. The use of master absorbent solutions and spectrophotometric data allowed for the preparation of absorption-based and scatter-based samples of known opacities. This ability allowed for the evaluation of the relative scatter-rejection capabilities of the system's two collimators. The MH collimator accurately measured highly-attenuating solutions of both absorption-based and scatter-based agents. The SS collimator experienced some contamination by scattered light with absorption-based agents, and significant contamination with scatter-based agents. Also, using the SS collimator, a `spiking' artefact was observed in highly-attenuating samples of both solution types. (ii): A change in imaging protocol has been described that greatly reduces ring artefacts that plagued the system previously. Scanning parameters related to the reference scan (Io) and data acquisition were evaluated with respect to image noise. Variations in flask imperfections were found to be a significant source of noise. (iii): An nPAG dosimeter was prepared, planned for, irradiated, and imaged using the fan-beam system. In addition to ring artefacts caused by data-corruption, refractive inhomogeneities and particulates in the gelatin were found to cause errors in image reconstructions. Otherwise, contour and percent depth dose comparisons between measured and expected values showed good agreement. Findings have indicated that significant imaging gains may be achieved by performing pre-irradiation and post-irradiation scans of dosimeters.

A radioterapia de intensidade modulada (IMRT) é uma das mais modernas técnicas radioterapêuticas que permite a entrega de elevadas e complexas distribuição de doses ao volume tumoral, que necessita de novos métodos para o controle de qualidade dos procedimentos efetuados. Nos serviços de radioterapia costuma-se usar para o controle de qualidade do sistema de planejamento, a câmara de ionização para verificação pontual da dose e um dispositivo com diodos semicondutores (MapCHECK2) para a verificação bidimensional em um plano da fluência planejada, entretanto, para a verificação tridimensional dessas distribuições de doses ainda não há um dosímetro consolidado na rotina clínica. Nesse contexto, para a dosimetria tridimensional se destacam os géis poliméricos. Neste trabalho foram feitas a dosimetria convencional, pontual e bidimensional como se faz na rotina clínica e a dosimetria tridimensional utilizando o gel polimérico Magic-f, que apresenta a distribuição de dose volumétrica. Para este trabalho foi escolhido o tratamento de câncer de próstata, pois na atualidade é um dos tipos de cânceres mais comuns entre os homens. No contexto da dosimetria gel, para se obter a informação volumétrica é necessária uma técnica de imagem, no presente caso foram utilizadas imagens por ressonância magnética (magnetic resonance imaging, MRI). A partir dessas imagens é possível determinar as distribuições de doses processando-as em um software desenvolvido pelo grupo que determina as taxas de relaxação R2 associada à dose absorvida e posteriormente comparar as imagens obtidas com as imagens do sistema de planejamento. Para isso, se obteve dez cortes ao longo de cada simulador físico ou fantom em que sua comparação foi feita com a respectiva fatia do sistema de planejamento, na posição correspondente. Para uma avaliação quantitativa foi utilizado o conceito de índice gama, no critério padrão da radioterapia, 3% da dose e 3mm de distância de concordância. Os resultados obtidos com a dosimetria gel se mostram de acordo com os controles de qualidade convencionais e oferecem uma visão global da distribuição de dose no volume alvo. / The intensity modulated radiotherapy (IMRT) is one of the most modern radiotherapeutic technique that enables the delivery of high and complexes conformational doses to the tumor volume, that requires new methods for the quality assurance of the procedures performed. Radiotherapy services usually perform quality assurance of the planning system with the ionization chamber for spot-checking and an array of semiconductor diodes (MapCHECK2) to check on a two-dimensional plane, however for tridimensional dose verification does not exist an established dosimeter in the clinical routine. In this context, for three-dimensional dosimetry the polymeric gels were used. In This work the conventional one and two-dimensional dosimetry as employed in the clinical routine, and the three-dimensional dosimetry using polymer gel MAGIC- f, which provide the volumetric dose distribution. Prostate cancer clinical cases were chosen for this work because this kind of tumor is one of the most common cases in male individuals. In the context of dosimetry gel to obtain volumetric information an imaging technique is necessary, in this case the magnetic resonance imaging (MRI), was used to measure the dose. From these images it is possible to determine the distributions of doses processing them in a software developed by our research group that determines R2 relaxation rates associated with the absorbed dose and subsequently compare the images obtained with the images of the planning system. For this, ten slices were obtained along each phantom, and comparisons were made with the respective slice of the treatment planning system, in the corresponding position. For a quantitative evaluation of the gamma index , in the standard criterion in radiotherapy, 3 % dose and 3 mm distance to agreement was used. The results obtained shown that gel dosimetry agrees with the conventional quality controls and provide an overview of dose distribution in the target volume.