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

[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.