Statistical process control of CBCT density gamma analysis as a clinical trigger for offline adaptive radiotherapy

Objective.Adaptive radiotherapy aimed to account for inter- and intra-fractional anatomical changes to improve target coverage and spare normal tissues. Commercial softwares could quantify anatomical and setup differences between planning computed tomography (pCT) and image-guidance cone-beam computed tomography (CBCT) using density gamma passing rate (dGPR). This study evaluated the utility of dGPR as an automated, site-specific trigger for offline adaptive workflows, using statistical process control (SPC) to define tolerance thresholds and correlating dGPR with setup errors, planning target volume (PTV) coverage, and longitudinal stability.Approach.240 patients across six anatomical sites were retrospectively analysed. First-fraction CBCTs were compared to pCTs using MobiusCB to compute dGPR. SPC analysis was performed to establish site-specific tolerance limits. Rigid phantom studies were conducted to quantify dGPR sensitivity to setup errors. Correlation between dGPR and PTVV_100%was assessed in patients with repeat CTs, and longitudinal stability of dGPR across fractions was evaluated.Main results.SPC-derived lower action limits (Al) ranged from 97.3% (brain) to 82.2% (breast), reflecting site-specific anatomical variability and imaging protocols. Phantom studies verified dGPR sensitivity due to rigid shifts, with head dGPR decreasing to as low as 83.5% at 8 mm translation, while pelvis dGPR remained above 93.2% for the same shift, reflecting greater tolerance due to its larger volume. In head-and-neck patients, dGPR correlated moderately with changes in PTVV_100%(r= 0.56), with no coverage losses >10% observed when dGPR exceeded 90%. Longitudinal analysis showed stable dGPR for most sites, but gradual declines in head-and-neck patients, with some values falling below SPC-derived threshold of 94.3% towards the end of treatment.Significance.dGPR offered a practical, automated, and site-specific metric for detecting anatomical changes and setup errors during radiotherapy. SPC-derived thresholds provided robust action levels tailored to each site, and MobiusCB enabled automated alerts when thresholds were exceeded, reducing reliance on subjective image inspection.