Simulação Monte Carlo para avaliação dosimétrica da influência de próteses metálicas no quadril em radioterapia de próstata

Abstract

Prostate cancer is the most common type of cancer among men, with the majority of cases being reported in individuals over 65 years of age. Radiotherapy is one of the treatment methods for this disease, aiming to treat the tumor with the lowest possible dose to the surrounding organs and tissues. However, patients undergoing radiotherapy for prostate cancer may have a hip prosthesis, implanted to replace the joint due to comorbidities such as osteoarthritis or rheumatoid arthritis, which are common in the elderly population. These prostheses may affect the dose distribution of the treatment, reducing the dose in the target volume and increasing the dose at the prosthesis-tissue interface. This study used the MCNP6.3 radiation transport code to simulate different treatment scenarios with a Versa HD linear accelerator at 6 MV. The hip prostheses were modeled using Blender 4.1 software and implanted into the male MRCP mesh phantom. The influences of stainless steel, CoCrMo alloy, and titanium alloy prostheses, placed unilaterally and bilaterally, were evaluated in two different field configurations for radiotherapy, through the determination of equivalent dose conversion coefficients (CC[HT]). Compared to simulations without prostheses, the stainless steel and CoCrMo alloy prostheses caused the greatest reductions in the 𝐶𝐶[𝐻𝑝𝑟𝑜𝑠𝑡𝑎𝑡𝑒] values. Bilateral prostheses resulted in greater reductions in the 𝐶𝐶[𝐻𝑝𝑟𝑜𝑠𝑡𝑎𝑡𝑒] values, with reductions of 8.76% (stainless steel), 8.65% (CoCrMo alloy), and 3.92% (titanium alloy), while unilateral prostheses caused reductions of 4.62% (stainless steel), 4.52% (CoCrMo alloy), and 2.21% (titanium alloy). When a field arrangement that does not pass through the prostheses was used, the dose delivered to the prostate was maintained at 100%, with average increases of 8.36% (unilateral prosthesis) and 11.06% (bilateral prostheses) for the 𝐶𝐶[𝐻𝑏𝑙𝑎𝑑𝑑𝑒𝑟], and 4.47% (unilateral prosthesis) and 4.28% (bilateral prostheses) for the 𝐶𝐶[𝐻𝑟𝑒𝑐𝑡𝑢𝑚]. Analysis of the electron tracks generated during treatment with a lateral 6 MV beam revealed a higher presence of electrons in the simulation with the unilateral stainless steel prosthesis, which can be explained by the backscatter produced by the high atomic number material. The results highlight the importance of adopting strategies to mitigate the impact of metallic prostheses in radiotherapy, ensuring the effectiveness of the treatment.