SU‐E‐T‐353: Feasibility of 3D Dosimetry for Prostate LDR: Monte Carlo Simulations of Pd‐103,I‐125, and Cs‐131 Seeds in Deformable PRESAGE

Abstract

Purpose: Recent innovations in 3D dosimetry include deformable polyurethane dosimeters (PRESAGE) and high resolution optical CT (∼50 microns). These developments enable 3D dosimetry in deformation conditions present during prostate LDR brachytherapy. However, water equivalence for the deformable dosimeter formulations has not been quantified. The purpose of this study was to characterize the dose distributions of Pd‐103, I‐125, and Cs‐131 in different PRESAGE using Monte Carlo (MC) simulations. Methods: Dose distributions from models 200(Pd‐103), 6711(I‐125), and CS‐1 Rev2(Cs‐131) seeds were simulated using the MCNP5 radiation transport. Geometric function g(r) and anisotropy function Φ(r) from TG43 were calculated from MC results in water and compared to published results to verify the MC technique. Dose distributions were then simulated in 5×5×10 cm^3 cylindrical PRESAGE with (0.1cm)^3 resolution. Current formulations of both conventional (rigid, 1.065 g/cm^3) and deformable (1.02 g/cm^3) PRESAGE with effective Z of 7.61 and 7.48 were simulated and g(r) and Φ(r) were calculated in X‐Y and X‐Z planes respectively. The 2×10^9 histories gave a typical statistical uncertainty (k=1) of ∼3% at 6 cm on the transverse plane. Results: Conventional PRESAGE over‐attenuated all three sources. Relative to water, g(r) in conventional PRESAGE decreased linearly ∼2.5%/cm within a radius of 1–6 cm for Pd‐103, and 1–10 cm for I‐125 and Cs‐131. Deformable PRESAGE under‐attenuated Pd‐103 but was water equivalent for I‐125 and Cs‐131, with g(r) within 5% and 1% of water within a radius of 0–10 cm, respectively. Differences for Φ(r) were <1% for Pd‐103 and Cs‐131, and <5% for I‐125 for both conventional and deformable PRESAGE. MC results indicated underdose >10% at dosimeter surface, while <2% with 2 cm backscatter. Conclusion: Dose distributions for Pd‐103, I‐125, and Cs‐131 have been characterized in PRESAGE and show great potential for water equivalent 3D dosimetry and further deformable formulations optimization. Research efforts at Duke University and Rider University were partially supported by National Institutes of Health (NIH) Grant No. R01 CA100835‐01. Dr. John Adamovics is the owner of Heuris Inc.