Intermediate rectal dose is associated with late toxicity after prostate SBRT: a dose–volume histogram principal component analysis

PURPOSE: To evaluate whether principal component analysis (PCA) of rectal and bladder dose-volume histograms (DVHs) identifies dose regions associated with late toxicity after prostate stereotactic body radiotherapy (SBRT). METHODS/PATIENTS: This retrospective single-institution study included 106 patients with linked clinical and dosimetric data after prostate SBRT to 36.25 Gy in five fractions. PCA was applied separately to whole-organ and wall-based rectal and bladder DVHs. Associations with late grade ≥ 2 gastrointestinal (GI) and genitourinary (GU) toxicity at 12 months were explored using Spearman correlation, ROC analysis and deliberately limited logistic regression. Toxicity was extracted retrospectively from institutional follow-up records scored according to CTCAE v5.0. RESULTS: Among 84 patients with 12-month follow-up in the DVH-linked dosimetric cohort, grade ≥ 2 toxicity was uncommon, with 3 GI events (3.6%) and 2 GU events (2.4%). Intermediate-dose rectal metrics showed the strongest exploratory signal for late GI toxicity, particularly rectal V18.1 Gy (AUC 0.868, 95% CI 0.740-0.997) and V29 Gy (AUC 0.827). These estimates are based on very few events and should not be interpreted as validated predictive performance or as evidence of a new planning threshold. No bladder or bladder-wall metric showed clinically meaningful discrimination for GU toxicity. CONCLUSIONS: Intermediate whole-rectum dose was associated with late GI toxicity in this low-event SBRT cohort, but the findings are exploratory and require validation. No isolated dosimetric predictor of late GU toxicity was identified.