SU‐E‐T‐582: Monte‐Carlo Study of Dosimetric Effects of Brachytherapy Seeds Presence in Lung External Beam SBRT

Purpose: In lungSBRT seeds left from a prior intraoperative brachytherapy may perturb the dose distribution near the target. We investigate the dosimetric effects of seeds present in the beam paths. Methods: BEAMnrc dosxyznrc code was used to generate 3D dose distributions for different seed arrangements using five 6 MV SBRT fields. A custom C++/Python code was developed to assign seeds of variable quantity and configuration to selected voxels with the Cs‐131 seed density. Two configurations of seed placement were considered: 1) an “odd” number of seeds per row, with the central column shadowing the isocenter, and 2) an “even” number of seeds per row without the isocenter shadowing. Results: For the “odd” configuration the isocenter dose decreased sharply with the number of seeds shadowing the isocenter (from 1.8% for 1 seed to 13.1% for 11 seeds). Adding more seeds in each row lowered the dose by additional 2.4% and this decrease was almost independent of the number of added seeds. A much smaller dose reduction (<1.8%) is observed for the “even” seed configuration, that does not shadow the isocenter directly. Conclusions: Dose reduction at the isocenter is proportional to the number of seeds that directly shadow the isocenter. Adding seeds in the beam path, without shadowing the isocenter, reduces the dose by a small amount. Since clinically the seeds are arranged in a mix of “odd” and/or “even” configurations with 1–2 rows and 1–2 columns, the isocenter dose is expected to be reduced by 0.6% to 5%.