Dose-volume analysis of planned versus accumulated dose as a predictor for late gastrointestinal toxicity in men receiving radiotherapy for high-risk prostate cancer

BACKGROUND AND PURPOSE: Significant dose deviations have been reported between planned (D(P)) and accumulated (D(A)) dose in prostate radiotherapy. This study aimed to develop multivariate analysis (MVA) models associating Grade 1 and 2 gastrointestinal (GI) toxicity with clinical and D(P) or D(A) dosimetric variables separately. MATERIALS AND METHODS: Dose volume (DV) metrics were compared between D(A) and D(P) for 150 high-risk prostate cancer patients. MV models were generated from significant clinical and dosimetric variables (p < 0.05) at univariate level. Dose-based-region of interest (DB-ROI) metrics were included. Model performance was measured, and additional subgroup analysis were performed. RESULTS: Rectal D(A) demonstrated a higher intermediate-high dose (V(30-65 Gy) and DB-ROI at 15–50 mm) compared to D(P). Conversely, at the very high dose region, rectal D(A) (V(75 Gy) and DB-ROI at 5–10 mm) were significantly lower. In MVA, rectal DB-ROI at 10 mm was predictive for Grade ≥ 1 GI toxicity for D(A) and D(P). Age, rectal D(A) for D(0.03 cc), and rectal D(P) for DB-ROI 10 mm were predictors for Grade 2 GI toxicity. Subgroup analysis revealed that patients ≥ 72 years old and a rectal D(A) of ≥ 78.2 Gy were highly predictive of Grade 2 GI toxicity. CONCLUSIONS: The dosimetric impact of a higher dose rectal dose in D(A) due to volumetric changes was minimal and was not predictive of detrimental clinical toxicity apart from rectal D(0.03 cc) ≥ 78.2 Gy for Grade 2 GI toxicity. The use of the DB-ROI method can provide equivalent predictive power as the DV method in toxicity prediction.