SU‐E‐T‐337: Monte‐Carlo Study of Dose Perturbation Effects in External Beam Therapy of the Prostate Caused by the Presence of Post‐Implant Brachytherapy Seeds

Purpose: Previously it has been reported that brachytherapydose distribution can be affected by seed self‐attenuation. We investigate the magnitude of dose perturbation in external beam radiation therapy caused by the presence of remaining brachytherapy seeds in the prostate. Methods: A phantom from a patient CT data was generated. A custom Python code was developed to modify the phantom by replacing selected voxels with the Cs‐131 seed material and density. 3D dose distributions and mean dose in the volume containing seeds for different seed arrangements were calculated using Monte‐Carlo EGSnrc dosxyznrc code and compared to the dose distributions without the seeds for a typical 5‐field treatment using 6MV and 18MV beams. Seeds were arranged symmetrically around the isocenter in a rectangular grid pattern with varying number of seeds along the x‐, y‐ and z‐ axes. In calculating mean dose and dose difference maps, seed voxels were excluded from calculation. Results: Difference in mean dose in the volume containing seeds and the same volume without seeds, was calculated for 6 and 18 MV beams. Mean dose increase is linear with the number of seeds and ranges from 0.7% for 75 seeds to 1.4% for 175 seeds for 6 MV and from 1.8% for 75 seeds to 4.6% for 18 MV for 175 seeds. Conclusions: In the external beam radiotherapy of the prostate, that contains seeds from the previous LDR seed implant, the mean dose in the prostate is increased proportionally to the number of seeds and the external beam energy used.