Avaliação dosimétrica de pacientes em terapia de próstata com braquiterapia e 3D-CRT utilizando o Método de Monte Carlo

Abstract

Radiation therapy is one of the most widely used treatments for patients suffering from certain types of cancer. However, the effectiveness of this treatment in destroying cancer cells is associated with the potential for experiencing effects of radiation in the surrounding healthy tissues. Technological advances allow for the optimization of treatments, reducing the administered doses and undesirable effects. In this context, the main objective of this study was to develop computational models to accurately calculate radiation doses in organs and tissues near and distant from the tumor volume in prostate cancer therapy patients. Two treatment modalities were evaluated: high dose rate brachytherapy and conventional photon beam radiotherapy produced in a medical linear accelerator. The patient was represented by an adult anthropomorphic phantom based on the ICRP 110 reference voxel, which was coupled with the radiation transport code, MCNPX2.7.0. This code was chosen for being one of the most widely used and accurate calculation programs in the field of neutron, photon, and electron transport simulations. The primary asymmetric results presented in this study are in the form of conversion factor. Analyses showed that therapy with 192Ir generated lower doses in the patient, especially for organs distant from the treated volume, due to the lower energy of photons compared to 3D-CRT. High conversion factor values for equivalent doses in organs near the prostate, such as testes, bladder, and colon, contributed to increasing effective doses in both evaluated modalities. Conversion factors for equivalent photon doses varied among selected organs, showing differences attributable to variations in LINAC computational models and anthropomorphic simulators. Regarding equivalent neutron doses, there was variation among organs. Generally, neutrons contributed less to the total patient equivalent dose in the 3D-CRT modality, except for organs such as the stomach, liver, lungs, esophagus, spleen, and thyroid, which were more affected due to exposure to moderate neutrons. Effective dose results for photons using 192Ir therapy and 3D-CRT were 8.4 and 12 mSv/Gy, respectively, which are similar to literature data, with 12 mSv/Gy for proton therapy and 13 mSv/Gy for IMRT. In the latter case, the conversion factor for effective dose originated from organs located outside the field of view, and therefore, real values are expected to be higher in clinical practice.