Evaluation of Patient Setup Accuracy using Optical Surface Guidance in Postoperative Radiotherapy for Patients with Breast Cancer after Breast-Conserving Surgery

Abstract Objective To evaluate the clinical application of AlignRT surface-guided radiotherapy (SGRT) by quantifying interfractional setup errors in patients with breast-conserving surgery (BCS) stratified by the surgical side and use of thermoplastic immobilization masks. Methods This prospective cohort study enrolled 125 patients with BCS treated at our center between January 2021 and June 2025. Patients were grouped by the surgical side (left/right) and immobilization method (with/without thermoplastic mask). After SGRT setup, cone-beam computed tomography (CBCT) was acquired for each fraction. Systematic and random errors were analyzed and three-dimensional planning target volume (PTV) margins were calculated. Results In total, 868 CBCT scans were obtained. With immobilization masks, left-sided patients exhibited significantly different setup errors from right-sided patients in the X (0.04 ± 0.25 cm vs –0.05 ± 0.30 cm, p < 0.001) and Y (–0.09 ± 0.26 cm vs 0.02 ± 0.30 cm, p < 0.001) directions. Without immobilization masks, differences persisted in X (–0.05 ± 0.22 cm vs 0.01 ± 0.27 cm, p = 0.017) and Y (0.03 ± 0.23 cm vs 0.08 ± 0.27 cm, p = 0.039). Irrespective of the side, mask use significantly affected the errors in all three directions (all p < 0.05). The required PTV margins (cm) were: left BCS with mask (0.51, 0.51, 0.54), and without mask (0.47, 0.49, 0.57); right BCS with mask (0.64, 0.63, 0.55), and without mask (0.59, 0.62, 0.53). Conclusion In SGRT-based workflows for patients with BCS, omitting the thermoplastic mask improves setup reproducibility. Right-sided BCS cases require larger margins than left-sided cases. Advances in knowledge This study provides clinical recommendations for patient positioning and immobilization techniques in whole-breast radiotherapy following breast-conserving surgery.