Post-radiotherapy prostate biopsies reveal heightened apex positivity relative to other prostate regions sampled

Background and purpose Prostate biopsy positivity after radiotherapy (RT) is a significant determinant of eventual biochemical failure. We mapped pre- and post-treatment tumor locations to determine if residual disease is location-dependent. Materials and methods There were 303 patients treated on a randomized hypofractionation trial. Of these, 125 underwent prostate biopsy 2-years post-RT. Biopsy cores were mapped to a sextant template, and 86 patients with both pre-/post-treatment systematic sextant biopsies were analyzed. Results The pretreatment distribution of positive biopsy cores was not significantly related to prostate region (base, mid, apex; p=0.723). Whereas all regions post-RT had reduced positive biopsies, the base was reduced to the greatest degree and the apex the least (p=0.045). In 38 patients who had a positive post-treatment biopsy, there was change in the rate of apical positivity before and after treatment (76 vs. 71%; p=0.774), while significant reductions were seen in the mid and base. Conclusion In our experience, persistence of prostate tumor cells after RT increases going from the base to apex. MRI was used in planning and image guidance was performed daily during treatment, so geographic miss of the apex is unlikely. Nonetheless, the pattern observed suggests that attention to apex dosimetry is a priority.