Assessing Axis-specific Set-up Errors in Rectal Cancer Radiotherapy: A Prospective Cone-beam Computed Tomography-Based Study with Body Mass Index Correlation.

Purpose: The purpose of the study was to quantify set-up errors and derive optimal clinical target volume to Planning Target Volume (PTV) margins for rectal cancer patients undergoing radiotherapy, while also evaluating the influence of body mass index (BMI) on set-up accuracy. Materials and Methods: Data from 41 patients and 1102 daily cone-beam computed tomography (CBCT) scans were analyzed. For each patient and fraction, the mediolateral (X), craniocaudal (Y), and antero-posterior (Z) translational shifts were measured. Population systematic (Σ) and random (σ) errors were calculated from per-patient summary statistics (see Methods), and PTV margins were derived using the van Herk formula (margin = 2.5 Σ +0.7 σ). Results: The mean systematic set-up error was small: 0.04 mm (X), 0.57 mm (Y), and 0.13 mm (Z), reflecting high reproducibility of daily image-guided positioning. Using Σ and σ, the derived PTV margins were 7.4 mm (X), 9.1 mm (Y), and 9.7 mm (Z). A moderate positive correlation between BMI and the Z-axis set-up error was observed ( Conclusion: Nonuniform, axis-specific margins are essential to accommodate anatomical and physiological variability in rectal cancer radiotherapy. The use of daily CBCT significantly enhanced set-up precision. Findings align with ICRU 62/83 and QUANTEC recommendations and support individualized planning approaches, especially in diverse patient populations where BMI and pelvic anatomy may affect positioning accuracy.