CT online adaptive radiotherapy is associated with dosimetric and acute toxicity improvements in prostate cancer treatment.

Abstract

Background and purpose: CT online adaptive (COA) radiotherapy allows for daily personalization of radiotherapy plans based on cone-beam CT imaging. Presently, the impact of COA on prostate cancer outcomes remains unknown.

Materials and methods: Records of 158 prostate cancer patients treated to 60 Gy/20 fractions in a single department with either COA with daily plan adaptation (n = 81) or non-adaptive radiotherapy (n = 77) were analyzed. Dosimetric changes resulting from COA were assessed. Short-term gastrointestinal (GI) and genitourinary (GU) toxicities were graded and logistic regression analyses with inverse probability treatment weighting (IPTW) were performed to assess the effect of COA on toxicities. The length of seminal vesicle tissue treated (LSV) was assessed as a predictor of COA outcomes.

Results: On multivariate analysis, COA was significantly associated with reduced GI toxicity (OR 0.45 95 %CI 0.27-0.73, p = 0.001) but not GU toxicity (OR 0.65 95 %CI 0.40-1.03, p = 0.07). Of 1620 COA fractions, 48.4 % demonstrated a clinically beneficial dosimetric change (63.7 %, 20.0 %, 15.1 % and 2.3 % for target coverage, rectum, small bowel and bladder, respectively) and 79/81 (97.5 %) of COA patients experienced at least one fraction exceeding a pre-defined threshold for clinical benefit. LSV was positively correlated with dosimetric benefit from COA (r = 0.42, p < 0.001). In non-adaptive patients, the LSV was associated with short-term GI toxicity (r = 0.30, p = 0.009) whereas this association was abolished with COA.

Conclusions: COA is associated with dosimetric improvements and decreased short-term GI toxicity. Accounting for daily variation in seminal vesicle position is a primary mechanism by which COA improves outcomes.