Radioactive sources used in brachytherapy must be dosimetrically characterized prior to clinical use as defined the TG-43 protocol. In our previous project, Gafchromic film dosimetry was used to experimentally obtain the anisotropy function for an M-19 iridium-192 brachytherapy seed being developed by Source Production Equipment Corp (St. Rose, LA). In this project, the Monte Carlo N-Particle Transport code (MCNP) was used to computationally obtain the full set of TG-43 parameters including the Dose Rate Constant, the Reference Dose Rate, the Radial Dose Function, and the Anisotropy Constant for the M-19 seed.

For patients receiving radiation therapy, there is a risk of developing radiation induced carcinomas, especially if they have a long life expectancy. However, radiotherapy is not the only contributor of radiation exposure to healthy tissue. With the introduction of highly conformal treatment techniques comes the increase in pretreatment imaging necessary to accurately target tumour volumes and consequently, radiation exposure to healthy tissue. In this work the radiation dose delivered to radiosensitive organs from a number of treatment planning techniques was evaluated and the risk of radiation induced cancer was assessed. MOSFET detectors and Gafchromic film were used to measure the accumulative concomitant dose to the thyroid and contralateral breast from early stage breast carcinoma radiotherapy and to the contralateral testis from seminoma radiotherapy, with dose contributions from CT imaging for treatment planning, pretreatment imaging (CBCT) and treatment delivery peripheral dose. To the author's knowledge this is the first work investigating the total concomitant treatment related dose and associated risk to these treatment sites. Peripheral dose contributed the largest concomitant dose to the healthy tissue, measuring up to 0.7, 1.0 and 5.0 Gy to the testis, thyroid and contralateral breast, respectively. The highest testicular, thyroid and contralateral breast carcinoma risk was found to be 0.4, 0.2 and 1.4%, respectively. In conclusion, the risk of radiation induced carcinoma to the assessed radiosensitive tissues was found to be minimal, however, when considering treatment techniques and/or introducing pretreatment imaging protocols, the dose to the normal tissue should be kept as low as reasonably achievable.

La précision du calcul de dose est cruciale pour la qualité de la planification et de la réalisation des traitements en radiothérapie. L’objectif de ce travail était d’évaluer la qualité des algorithmes de calcul de dose des faisceaux d’électrons pour des conditions particulières proches des situations cliniques rencontrées. Une méthodologie spécifique de dosimétrie par films radiochromiques des faisceaux d’électrons a été développée et validée, y compris pour des situations difficiles avec une précision de 3.1% en dose absolue et 2.6% en dose relative. Cette technique a permis de développer des tests de Contrôle Qualité généralistes rapides et efficaces qui servent de base de données mesurées en cas de changement de version du système de planification de traitement et/ou d’algorithme de calcul de dose. Les mesures par films ont été comparées avec les calculs de deux algorithmes de calcul de dose Pencil Beam et Monte Carlo. Les deuxcodes ont donné des résultats similaires dans les tests d’obliquité, d’irrégularité et de DSP étendue. En revanche, les calculs Monte Carlo sont plus précis en présence d’hétérogénéités. D’autre part, cette méthode de dosimétrie par films radiochromiques a permis de développer un nouveau mode d’évaluation des plans de traitement avec des films découpés et insérés dans des fantômes anthropomorphiques (de type thorax et tête) de manière à obtenir la distribution de dose en 2D dans un plan transversal donné ; ces tests cliniques pouvant aussi être utilisés dans le cadre d’un Contrôle de Qualité interne adapté à l’activité clinique du service. La comparaison des mesures avec les calculs a montré une meilleure précision dans les calculs Monte Carlo par rapport aux calculs Pencil Beam au niveau des hétérogénéités, notamment dans les poumons, les cavités et les os. / In radiotherapy, the dose calculation accuracy is crucial for the quality and the outcome of the treatments. The purpose of our study was to evaluate the accuracy of dose calculation algorithms for electron beams in situations close to clinical conditions. A new practical approach of radiochromic film dosimetry was developed and validated especially for difficult situations. An accuracy of 3.1% and 2.6% was achieved for absolute and relative dosimetry respectively. Using this technique a measured database of dose distributions was developed to form the basis of several fast and efficient QualityAssurance tests. Such tests are intended to be used also when the dose calculation algorithm is changed or the Treatment Planning System replaced. Pencil Beam and Monte Carlo dose calculations were compared to the measured data for simple geometrical phantom setups. They both gave similar results for obliquity, surface irregularity and extended SSD tests but the Monte Carlo calculation was more accurate in presence of heterogeneities. The same radiochromic film dosimetry method was applied to film cuts inserted into anthropomorphic phantoms providing a 2D dose distribution for any transverse plan. This allowed us to develop clinical test that can be also used for internal Quality Assurance purposes. As for simpler geometries, the Monte Carlo calculations showed better agreement with the measured data than the Pencil Beam calculation, especially in presence of heterogeneities such as lungs, cavities and bones.