SU‐E‐T‐255: Monte Carlo Quantification of Dose Distributions Around Thin Lead Foils

Abstract

Purpose: Patients are increasingly undergoing radiotherapy procedures, in which small metals are implanted in the body for target localization in IGRT or dose enhancement. Previously we proposed the polymer gel dosimetry technique for measurement of 3D dose distributions around small metallic materials including thin foils. The purpose of this study is to determine the effect of separation between thin metallic objects and 3D dose distributions using a Monte Carlo method and to provide theoretical data which will be compared with the measurement data. Methods: DOSxyz was used to perform Monte Carlo simulations of a two lead foil setup. The Mohan 24MV spectrum was used as a radiation source with 5cm x 5cm collimation at a 100 SSD. The foils had dimensions 1cm × 1cm × 0.5mm and had separations of 2mm, 5mm, 7mm, 10mm and 15mm. The phantom size was 16cm × 16cm × 28cm. Simulations were performed on custom Intel I7 machines with various histories (100 million to 1 billion). Percent depth dose curves were determined, then the minimum dose between the foils was recorded and compared to the dose without the foil. The plots were normalized against the maximum dose point. Results: The minimum dose enhancement ratios between two foils were 1.47, 1.29, 1.18, 1.13 and 1.08 for the separation of 2, 5, 7, 10, and 15 mm, respectively. The dose enhancement ratio exponentially decreased with the separation. The Monte Carlo simulation results were in agreement with experimental data previously obtained with films. Conclusion: The degree of dose enhancement around thin lead foils was determined by the Monte Carlo method. It is expected that other high Z materials such as gold will behave in similar manner. The results would be used to identify optimal placement of materials for measurement by a polymer‐gel dosimeter.