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

[BACKGROUND 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.