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A typical cervical cancer treatment includes primary external beam radiation therapy followed by a course of high dose-rate Intra-Cavitary brachytherapy using Flethcher-Suit-Delclos (FSD) applicator. Current brachytherapy planning system does not account for inhomogeneities, neglecting the attenuation effect from applicator material, and leading to potential inaccuracies in computation of dwell times and dose distribution. The purpose of this study is to quantify the effect of 192Ir source attenuation due to titanium material used in the new CT-compatible FSD applicator during treatment delivery.For this study a detailed geometry of the applicator obtained from the manufacturer was implemented into a Monte Carlo (MC) model using simulation package MCNP5. A tandem with bending angle of 15 degrees was used for a sample simulation while colpostats presented here have bending angle of 120 degrees. The source geometry is of the VariSource wire model VS2000. We assigned eight dwell positions in the tandem and four dwell positions in each colpostat to calculate dose rate at reference points A and B with and without titanium wall present in the simulation.Based on the MC computation, the titanium tube reduced the overall dose to point A and point B by ~2.97%, with contributions varying for each dwell position. We also compared MC results with the BrachyVision commercial treatment planning system calculations. The clinically used algorithm is based on AAPM report TG-43, which calculates the dose without any inhomogeneity correction. The latest Varian release of the planning software BrachyVision-Acuros has the updated algorithm capable of inhomogeneity corrected dose calculation. For this study a treatment plan was created based on the exact coordinates of our MC model and used to calculate dose at reference points. The dose to reference point A and point B with the traditional BrachyVision algorithm and with BrachyVision Acuros is in excellent agreement with the homogeneous MC model result. The attenuation due to Ti wall is ~2.83% at point A compared to ~5% using a non-CT compatible FSD system. Comparative accuracy of the two treatment planning systems with MC, especially in their abilities to account for the source attenuation due to the applicator titanium walls is presented in this study. |
