Introduction of Monte Carlo dosimetry and edema in inverse treatment planning of prostate brachytherapy

Abstract : Prostate cancer is the second most common cancer in men. Two-thirds of the cases are diagnosed in developed countries and France is ranked third in incidence rate. Low-dose-rate (LDR) brachytherapy is a widely used treatment option. During LDR brachytherapy, radioactive seeds are implanted permanently in the prostate to deliver a therapeutic dose locally in the cancerous region while sparing the organs at risk (OARs). Despite its high success rate (75% to 91%), the side-effects (sexual and urinary problems) remain high. The dose delivered to the tumor depends on the implantation positions of the seeds, which implies that treatment planning is essential. Clinical inverse planning systems automatically provide optimal implantation positions. However, this prediction is based on a simplified dosimetric model where the human body is considered an infinite volume of water. Another important factor that induces treatment errors is the occurrence of prostate edema during brachytherapy that involves volumetric changes of the organ. Edema can lead to a significant underestimation of the D90, for example, by 13.6% for a volumetric change of 20%. Moreover, the edema magnitude varies considerably (10% to 96%) between patients. Today the exact mechanism of edema formation remains unknown. We propose in this thesis a system of inverse treatment for prostate brachytherapy which considers a precise personalization of the dosimetry but also of the prostate edema. This work can also be used in other clinical contexts, such as high-dose-rate brachytherapy, but also be adapted to treat other organs. In the future, our work will focus on the study of the ability to adapt the proposed prostate biomechanical model to each patient using elastic measurements via prostate elastography. Due to the inherent limitations of FEM, the incorporation of the biomechanical model of edema into the treatment planning system is costly in computation time. An alternative method would be to propose a new meshless model to improve the simulation of edema during intraoperative planning.
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Konstantinos Mountris. Introduction of Monte Carlo dosimetry and edema in inverse treatment planning of prostate brachytherapy. Human health and pathology. Université de Bretagne occidentale - Brest, 2017. English. ⟨NNT : 2017BRES0061⟩. ⟨tel-01688367⟩

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