Surface-guided versus conventional setup in rectal cancer radiotherapy: A workflow-level comparison of CBCT-Derived setup corrections.

BACKGROUND: Precise patient positioning is essential in rectal cancer radiotherapy to ensure target coverage while protecting organs at risk. Surface-guided radiotherapy (SGRT) is a non-invasive setup method that may improve accuracy and reduce repeat imaging. This study compared SGRT-based positioning with conventional tattoo-based setup. METHODS: This retrospective analysis included 100 rectal cancer patients treated with Volumetric Modulated Arc Therapy on Varian linear accelerator: TrueBeam (SGRT-guided positioning) and Halcyon (manual Delta couch positioning). Translational shifts from CBCT verification were analysed across all axes, and additional CBCT imaging at matched fractions was compared (p ≤ 0.05). RESULTS: Mean translational corrections were numerically larger on the Halcyon in five of six directions, but none of the six between-accelerator comparisons remained statistically significant after correction (corrected p ≥ 0.744; effect size r ≤ 0.15). Within accelerators, TrueBeam corrections were symmetric across all three axes, whereas the Halcyon showed a significant left-right asymmetry (p = 0.031). At least one additional CBCT scan was required in 48% of Halcyon patients versus 18% of TrueBeam patients (p = 0.001), and the mean number of additional scans was more than seven times higher on the Halcyon (1.40 vs 0.18; p < 0.001), driven mainly by bladder and rectal filling rather than gross positional error. CONCLUSION: Translational setup accuracy was comparable between workflows, but Halcyon patients required substantially more repeat CBCT imaging, suggesting that SGRT's main benefit lies in reducing imaging burden rather than improving geometric accuracy.