3186 - Is there an association between the location of the dominant intraprostatic lesion and biochemical/clinical failure following treatment with LDR prostate brachytherapy?
O. A. Houlihan1,2, S. Esteve2, O. McLaughlin1, G. Workman2, M. Byrne2, E. Napier2, K. M. Prise1, A. Hounsell2, D. M. Mitchell2, and S. Jain1,2; 1Patrick G. Johnston Centre for Cancer Research, Queens University Belfast, Belfast, United Kingdom, 2Northern Ireland Cancer Centre, Belfast, United Kingdom
Purpose/Objective(s): Dose in LDR prostate brachytherapy is routinely prescribed to the global prostate which has correlated with outcome in some studies but not in others. Dose to prostate sectors has wide variation with anterior base sectors typically receiving a lower dose. To date no study has examined dose to the dominant intraprostatic lesion (DIL) sector to see if it predicted freedom from biochemical/clinical failure (BCF). Materials/
Methods: We examined a cohort of 606 patients treated with LDR brachytherapy (145Gy monotherapy n=546; 110Gy combination n=60) in a single institution from 2009-20 (13% low, 80% intermediate, 7% NCCN high-risk). The prostate sector(s) in which the DIL was located, was/were identified following review of the diagnostic MRI (n=599) and pathology report (n=606). Outcomes were blinded during this process. The prostate was divided into 12 sectors; 3 equal thirds (base/midgland/apex), and 4 axial sectors by two separate methods; plus (“+”) and cross shape (“x”). Kaplan-Meier curves were generated with the log-rank test performed to assess for differences in BCF rates. Results: Relative to the global prostate gland (D90 108.0%, IQR 101.8-114.3%), the anterior base received the lowest median (IQR) D90 (85.9%, 76.6-95.6%) and the right and posterior midgland received the highest median D90; 142.2% (131.1-153.7%) and 141.8% (128.3-155.5%) respectively. 5-year BCF for the entire cohort was 89.6%.Using cross-shaped prostate sector division, freedom from BCF was significantly lower when the DIL was located in the left and right base (71.3%, p=0.0004 and 75.0%, p=0.001 respectively) and higher when the DIL was located in the posterior midgland (96.8%, p=0.002) with HR 0.4 (95% CI 0.2-0.7) relative to when the DIL was located elsewhere. Using plus-shaped prostate sector division, the anterior right and left base sectors had significantly lower BCF event free rates (76.7%, p=0.004 and 49.1%, p=0.0002 respectively) with respective HR 5.4 (1.7-17.0) and 9.6 (2.9-31.3). Conclusion: This is the first LDR brachytherapy study to analyze biochemical outcomes according to the location of the DIL. The lower BCF event free rates when the DIL was located in the anterior base may be due to the lower dose received due to its anatomical location, which can be difficult to implant due to pubic arch interference and proximity to the bladder neck, resulting in lower planned dose to this region. These findings reinforce the importance of considering the location of the DIL when determining the suitability of patients for prostate brachytherapy. The higher BCF event free rates where the DIL was located in the posterior midgland may be related to the higher dose delivered to this sector relative to the global prostate gland. These findings suggest dose-escalation to the DIL in LDR prostate brachytherapy may improve patient outcomes in the treatment of prostate cancer.
