Longitudinal voxel-based analysis identifies a bladder neck subregion predicting genitourinary toxicity after prostate cancer reirradiation

AbstractBackground and purpose In the context of prostate cancer reirradiation, stereotactic body radiation therapy (SBRT) may allow to spare anatomical volumes potentially implicated in genitourinary (GU) toxicities. This work aimed to perform a longitudinal voxel-based analysis to explore urethrovesical subregions where a higher planned dose was associated with an increased risk of late GU toxicity. Material and methods A single-centre cohort of 108 local prostate cancer recurrence patients treated with salvage SBRT (36 Gy / 6 fractions) and followed-up for a median period of 50 months was considered for this retrospective study. A non-rigid registration was applied to align 3D dose distributions of the entire population in a common space. A multivariate voxel-based analysis using Cox regression was performed on aligned doses and clinical parameters to identify subregions where a higher dose was associated with an increased risk of toxicity. Dose-volume histograms (DVHs) differences between patients with and without GU toxicity from 8 to 72 months after SBRT were computed in organs at risk (OARs) and the identified subregions. The DVH predictive capabilities were also tested. Results A subregion where a higher planned dose was associated with an increased risk of grade ≥ 3 GU toxicity was identified in the bladder neck, at the intersection with the internal urethral sphincter. The subregion DVH demonstrated superior predictive performance (AUC > 0.8) compared to those of the entire OARs. Conclusion An anatomical subregion where the planned dose was predictive of late grade ≥ 3 GU toxicity was defined in the bladder neck.