SU‐F‐19A‐07: Is a Day30 Scan Necessary to Evaluate Activity‐Based Regulatory Compliance in Permanent Interstitial Brachytherapy for Prostate Cancer?

Abstract

Purpose:To evaluate the Medical Event (ME) criteria for I‐125 prostate implants based on the assessment of post implant dosimetry on “Day0”/“Day30” imaging. The new ME criteria do not mandate a timeframe for this assessment. The compliance criteria are: more than 80% of the activity from the written directive for treatment site (TS) must be implanted inside TS, and doses to 1cc of either uninvolved rectum (D1_UR) or uninvolved bladder (D1_UB), or 2cc of other non‐specified tissue (D2_UT) must be less than 150% of the planned dose.Methods:“Day0”/“Day30” post‐implant analyses for 25 patients were evaluated. Treatment plans had a peripheral loading pattern with 2 core needles placed at least 10 mm away from urethra, with several seeds planned outside of the prostate for adequate target coverage. TS were a uniform 5 mm expansion of the prostate, except posteriorly (no expansion).Results:“Day0”/“Day30”analyses found no MEs. The relative changes for D1_UR, D1_UB, and D2_UT were (ranges): [−37.0, 38.2]%, [−96.5, 74.7]%, and [−41.2, 37.7]%. Furthermore, changes did not correlate with prostate volume changes of −18.7% [σ:16.0%, range:−60.5%, +6.4%]. These unfavorable changes did not lead to ME at “Day30” because these values were generally well below 150% at “Day0”. However, D2_UT dose values exceeded those for D1_UR and D1_UB at both “Day0”/“Day30”.Conclusion:The total activity was relatively insensitive to changes in target volume from “Day0” to ”Day30”. The dose metrics of interest, albeit susceptible to large, often unfavorable changes, remained less than the 150% threshold. Data from this study suggest that “Day0” can be used for the regulatory compliance evaluation. However, further evaluation at “Day30” is advisable if D2_UT is 110% or above (based on the largest D2_UT increase of 37.7% observed in this patient population). Future rigorous statistical analysis of a larger cohort will afford a refinement of this recommendation.