Dosimetric impact of CBCT-guided online adaptive stereotactic body radiotherapy for prostate cancer: a 250-fraction retrospective analysis.

Purpose

To quantitatively evaluate the dosimetric benefit of CBCT-guided online adaptive radiotherapy (ART) for prostate SBRT compared with non-adaptive treatment planning, with emphasis on target coverage preservation and organ-at-risk (OAR) sparing.

Methods

AND MATERIALS Fifty prostate patients treated on the Varian Ethos platform were retrospectively analyzed. For each fraction, scheduled plans (reference plan recalculated on daily anatomy) were compared with adapted plans (daily reoptimized plans). Target coverage was assessed using PTV D95% while bladder and rectal metrics were evaluated using institutional constraints. Fraction-level paired comparisons used the Wilcoxon signed-rank test. Fractions with substantial scheduled-plan under-coverage (PTV D95% ≤80%) underwent cumulative dose reconstruction.

Results

Adaptive plans achieved higher target coverage than scheduled plans (p < 0.001), although scheduled plans remained within ∼3% of prescription in 83% of fractions. Scheduled plans demonstrated a 20% reduction in PTV D95% in 3.6% of fractions and >5% coverage reduction in 11%, whereas no adaptive fractions showed similarly large deviations. Cumulative scheduled-only dose remained near prescription for most affected patients, with only one patient demonstrating a cumulative D95% reduction of ∼5%. OAR differences were small overall, with ART planning mainly reducing occasional constraint violations. Strict application of planning objectives resulted in 234 adaptive fractions meeting goals versus 62 scheduled fractions, improving to 135 with offline renormalization.

Conclusions

CBCT-guided ART for prostate SBRT primarily improves robustness against infrequent but clinically significant dosimetric deviations rather than providing consistent superiority across most fractions.