SU‐E‐T‐448: Heightened Apical Positivity of 2‐Year Post‐Radiotherapy Biopsies Is Not Related to Suboptimal Dosimetry

Abstract

Purpose:Previous research has demonstrated that following radiation therapy for prostate cancer, there is a relative increase in positive biopsies in the apex versus the rest of the prostate. The increase could be due to: 1) Inter‐fraction apex motion or deformation, 2) Intra‐fraction apex motion or deformation, 3) Suboptimal dose coverage in the apex, 4) Tissue composition in the apex and/or 5) Prostate size. In this initial study, the potential for suboptimal dose coverage in the apex was assessed by splitting the prostate planning target volume into the apex (inferior third) and remainder.Methods:69 patients were selected from 303 patients treated on a clinical radiotherapy trial for prostate cancer. These patients were selected as they had both a localized (sextant template) 2‐year post‐treatment biopsy and 3D dose information. Of these patients, 10 had positive biopsies in the apex, 8 in the remainder and 11 in both locations. For all patients, the following dosimetric data was acquired from the apex dose volume histogram: Dmean, Dmax, Dmin, D95% and V100%. Unpaired, one‐tailed t‐tests were used to test for statistical significance (p < 0.05) between all dosimetric parameters for patients with positive versus negative apical biopsies. Additionally, D95% for the apex was plotted against D95% of the remainder.Results:There was no statistical difference for the selected apical dosimetric parameters for patients with positive versus negative biopsies (p‐values > 0.05). No correlation was found between D95% (normalized to the prescription dose) for the apex and remainder (R2 = 0.0116).Conclusion:No correlation was found between positive apical biopsy and suboptimal dosimetric coverage. Current research is looking into inter‐fraction apex motion and deformation as a potential source of the increased apical failure using daily CBCT images.