Effect of Differences in Insert Materials of Cutout Block on Dose Distribution in Electron Radiotherapy: Comparison between Measurements and Monte Carlo Simulation

Abstract

In electron beam radiotherapy, an irradiation field is created with a cutout block using a low melting point lead alloy. The block can be replaced with a lead plate as a shield. Dose distribution is expected to be affected by differences in the material and thickness of the shield. Thus, this study aimed to investigate the cause of differences in dose distribution by reproducing the electron beam irradiation condition via Monte Carlo simulation, comparing dose distribution when each shield is used and analyzing energy fluence distribution. Radiation interaction in the treatment device manufactured by Varian was assessed using the general-purpose simulation code, and the dose distribution in the water was calculated. Electron energy fluence and incident angle of the electron fluence incident on the water surface were analyzed, and the effect of the difference in the shield was investigated in the irradiation field limited to 3 cm or less. Regarding dose distribution, the deviation in the buildup area became larger when the lead plate was made thinner. A difference of 1.6-6.8% was observed on an average when comparing the buildup region of depth dose distributions except for 1×1 cm2 field. In electron energy fluence, the lower the lead thickness, the higher the low energy component, which affected the buildup region. The effect was greater as the electron beam energy increased. It was possible to evaluate the difference in scattered radiation between the low melting point lead alloy and the lead plate by MC simulation. Based on the study findings, the effect of scattered electrons generated from the block was strong as a factor.