Analysis of set-up errors in daily kilovolt fan beam CT for rectal cancer treated with volumetric modulated arc therapy

To analyze inter-fractional set-up errors in rectal cancer patients undergoing volumetric modulated arc therapy (VMAT) guided by daily kilovolt fan beam CT (kV-FBCT). A total of 720 set-up error measurements were collected from 30 rectal cancer patients treated with VMAT. Translational (VRT, LNG, LAT) and rotational (yaw, pitch, roll) errors were extracted from daily kV-FBCT image registration. PTV margins were estimated using the van Herk formula. We performed regression analysis to identify factors influencing the magnitude of set-up error and conducted time trend analysis to evaluate variations in set-up errors across treatment fractions. The study revealed a mean center migration of 0.93 ± 0.47 cm, with corresponding PTV margin expansions of 0.86 cm (VRT), 1.43 cm (LNG), and 0.83 cm (LAT). Regression analysis identified a trend toward female gender and height as potential predictors for set-up errors in the LNG and pitch directions. Importantly, no significant time trend was observed for set-up errors across treatment fractions. At the patient level, early-fraction (2–5) and late-fraction (6–25) set-up errors showed no significant differences, suggesting that early imaging data may help refine setup for subsequent fractions. However, individual predictions should be interpreted cautiously, as anatomical and positioning variability remain patient-specific. For centers not using daily FBCT or CBCT, our findings suggest that extending PTV margins in the LNG direction may improve target coverage for rectal cancer patients receiving VMAT. Patient-specific factors such as female sex and greater height were associated with increased susceptibility to set-up errors, warranting closer monitoring in these subgroups. While early-fraction imaging data may help optimize positioning for subsequent fractions, individualized adjustments should be made cautiously. • Quantified interfractional set-up errors using daily kV-FBCT in rectal cancer. • Analyzed 720 FBCT datasets from 30 patients over 24 treatment fractions each. • Conducted on a uRT-linac 506c platform with an All-in-One radiotherapy workflow. • Female sex, height, and weight were predictors of LNG and pitch deviations. • Early-fraction set-up errors can guide remaining treatment positioning.