Accumulated dose on daily iCBCT for rectal cancer: Effects of inter‐fraction bowel cavity motion volume

Abstract Background To assess how inter‐fraction bowel cavity (BC) motion volume influences accumulated dose in rectal cancer radiotherapy and to translate these effects into a clinically interpretable risk construct for acute bowel toxicity. Methods This retrospective cohort included 28 consecutive rectal cancer patients treated with 50 Gy in 25 fractions on a Halcyon v3.0 linac with daily iterative cone beam CT (iCBCT). Patients with > 5% bladder‐volume or > 8% external‐contour variation across the course were excluded. For each fraction, per‐fraction dose was recalculated on the iCBCT using a scanner‐specific HU‐relative electron density calibration, rigidly aligned to the planning CT (pCT), deformably registered, and voxel‐wise accumulated on pCT. Dosimetric endpoints were PTV D 2% , D 50% , D 98% and whole‐bowel (WB) D 0.03cc , D 150cc . Associations between inter‐fraction BC motion volume change (ΔV BC ) and accumulated dose difference (ΔD) were quantified using Spearman's f and ordinary‐least‐squares slopes (Gy per 10% BC expansion); same‐day effects were estimated with patient fixed‐effects models. To account for multiple testing, p ‐values for the five primary endpoints were adjusted using the Benjamini–Hochberg false discovery rate (FDR). Clinical relevance was assessed with a published logistic normal‐tissue complication probability (NTCP) model based on WB V 45 , defined as the WB volume receiving ≥ 45 Gy. Results Accumulated dose analyses showed selective elevation of high‐dose metrics with preservation of near‐minimum target coverage. Compared with the plan, accumulated PTV D 2% and D 50% increased by 0.87 Gy and 0.61 Gy, respectively, whereas PTV D 98% exhibited no meaningful change. WB D 0.03cc and D 150cc increased by 1.21 Gy and 1.18 Gy, respectively. Inter‐fraction BC expansion was strongly associated with high‐dose escalation: for each 10% increase in BC volume, accumulated WB D 0.03cc increased by 0.31 Gy (95% CI 0.18–0.44; ρ = 0.78) and PTV D 2% by 0.29 Gy (0.15–0.37; ρ = 0.66). After FDR adjustment, no endpoint met p < 0.05, but the strongest associations (PTV D 2% , WB D 0.03cc ) remained near‐significant and directionally consistent with large, biologically plausible effect sizes. Application of the WB V 45 NTCP model indicated modest but consistent risk increments after accumulation, with NTCP for ≥ grade 2 toxicity increasing from 0.75 to 0.79 (+0.04; p = 0.001) and NTCP for grade 3 toxicity increasing from 0.80 to 0.84 (+0.04; p = 0.002). Conclusion Daily iCBCT‐based dose accumulation identifies inter‐fraction BC motion volume as a selective driver of high‐dose escalation to the PTV and WB while largely preserving near‐minimum target coverage. Accumulated WB V 45 may serve as a practical, delivery‐based marker for on‐treatment surveillance and risk‐adapted intervention, complementing plan‐based evaluation in rectal cancer radiotherapy.