Room: Exhibit Hall
Purpose: To evaluate dosimetric changes from source migration and decay for three different seed radioisotopes when imbedded in a collagen carrier/spacer in brain brachytherapy.
Methods: Seven patients undergoing Cs-131 implantation and subsequent IGRT on an IRB-approved trial were identified. Seed positions (avg.18/implant) were manually defined on 47 image sets acquired 7-57 days post-implant and for 3 additional image sets acquired at 82-289 days. In addition to the clinical post-implant Cs-131 plans, hypothetical post-implant plans were generated for I-125 and Pd-103 utilizing the same seed positions as Cs-131 plans while modifying seed strength to match the Cs-131 60Gy brachytherapy volume. To model the impact of seed migration and decay, plans were recalculated for 2 scenarios: 1) assuming seed positions on a given image set were unchanged until acquisition of the subsequent set, and 2) assuming seed positions had changed on the day after the prior image set was acquired.
Results: For scenarios 1 and 2, respectively, the average mean dose difference to the 60Gy volume compared to the post-implant plans, was 0.4Gy (0.6%, range 0.1-1.0Gy) and 0.9Gy (1.5%, range 0.2-1.7Gy) for Cs-131, 1.2Gy (2.0%, range 0.1-2.1Gy) and 1.6Gy (2.6%, range 1.2-2.6Gy) for I-125, and 0.8Gy (1.3%, range 0.2-1.5Gy) and 1.4Gy (2.3%, range 0.3-1.9Gy) for Pd-103. Differences in V100, V150 and V200 on average were <0.3cc for all 3 radioisotopes. Differences in V30 and V15 were <0.3cc and <0.4cc for Cs-131, <0.4cc and <0.6cc for Pd-103, and <0.7cc and <1.2cc for I-125. Inclusion of extended time points for I-125 plans did not result in significant changes from the above values.
Conclusion: The largest dosimetric impact from seed migration was for I-125 and the least for Cs-131. Observed dose differences from seed migration are relatively small across all three radioisotopes and may not be clinically significant, implying the robustness of the implantation technique.