Clinical indicators for dose re-optimization in magnetic resonance-guided online adaptive radiotherapy for prostate cancer

Background and purpose This study aimed to identify dose-volume parameters and anatomical features associated with the implementation of online dose re-optimization in magnetic resonance-guided adaptive radiotherapy (ART) for prostate cancer and to evaluate its impact on target coverage and organs-at-risk dose-volume parameters.Materials and methods Treatment plans from 150 fractions (with and without dose re-optimization) in 30 patients receiving five-fraction stereotactic body radiotherapy were retrospectively analyzed. Evaluated parameters included planning target volume (PTV) V 100%, bladder and rectum dose-volume parameters, volumetric changes, and relationship between changes in target coverage.Results Online dose re-optimization was performed in 60% of fractions and became more frequent in later fractions (p = 0.02). Compared with reference plans, non-ART plans showed a 3.7% lower median PTV V 100% (p p V 100% in non-ART plans, while this relationship was substantially weakened after dose re-optimization. Although bladder and rectum dose-volume parameters increased significantly in non-ART plans, all median values remained within predefined dose constraints.Conclusions Progressive target-volume expansion appears to be the primary anatomical factor associated with deterioration in target coverage. A reduction in PTV V 100%, particularly below 95%, represented a clinically meaningful indicator for online dose re-optimization. These findings suggested that online dose re-optimization remained important for maintaining target coverage and limiting gradual deterioration of organs-at-risk dose-volume parameters, even when predicted plans satisfied clinical constraints.