Monte Carlo simulations of the dose distribution of heterogeneous media irradiated with small fields of flattening filter free photon beams

• Main Authors: Chien, Yi-Chen, 簡宜蓁
• Other Authors: Chang, Kwo-Ping
• Format: Others
• Language: zh-TW
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/b89twh

碩士 === 慈濟科技大學 === 放射醫學科學研究所 === 107 === Due to the dose perturbation caused by the small field size and the heterogeneous media, the algorithm of the treatment planning system (TPS) may evaluate the radiation dose inaccurately. The Monte Carlo simulation program, BEAMnrc and DOSXYZnrc, have been used to simulate the dose distribution and perturbation of the heterogeneous media in the small field sized 6 MV flattening filter free (FFF) photon beams delivered by Varian TrueBeam. The results of the Monte Carlo simulations were compared with those calculated by two Eclipse v13.6 TPS algorithms, anisotropic analytical algorithm (AAA) and Acuros XB (AXB). It was found that the highest overestimation of AAA and AXB values in the air cavity were 353.2 % and 88.9 % respectively for all of the small/narrow field sizes. Additionally, the narrow field size 1 × 10、 10 × 1 cm2 have similar characteristics as their equivalent small square field size (1.81 × 1.81 cm2). However, due to secondary electron compensation from the long-side (10 cm), the inaccuracy estimated by AAA/AXB for narrow field sizes is smaller than that for the equivalent square small field size. The significant inaccuracy was found for the dose profile in the air cavity for 1× 10 cm2 field size using AXB algorithm, maybe it is because the TPS lacks of the beam data for 1 cm field size. Throughout the overall comparisons, the calculations by the AXB algorithm can reflect the rebuild-up and the dose phenomenon as the Monte Carlo simulations. It is strongly recommended that the AXB algorithm is the preferred option of the stereotactic radiosurgery (SRS) / stereotactic body radiation therapy (SBRT) treatment planning for small field size, to improve the accuracy of assessing the dose distribution in the heterogeneous media/interfaces.