3018 - Comparative Analysis of Optical Surface Monitoring System Automatic Positioning Function and Conventional Landmark Positioning in Breast Cancer Using Radiotherapy

Purpose/Objective(s): This study compares two positioning approaches for breast cancer patients undergoing breast bracket fixation: conventional surface landmark positioning and automatic positioning utilizing the optical surface monitoring system (OSMS). The primary focus is on their different impacts on therapeutic precision, and to derive adapted CTV-to-PTV margins.

Materials/Methods: A retrospective analysis was conducted on 52 breast cancer patients treated between June and November 2023. Patients were stratified based on positioning techniques into two groups: a conventional surface marker-based group (n=26) and an optical surface monitoring system (OSMS) group (n=26). Each group was further subdivided by respiratory motion management strategy into deep inspiration breath-hold (DIBH) and free-breathing (FB) subgroups (13 cases each). Six-dimensional registration errors were acquired via cone-beam computed tomography (CBCT) to evaluate positional discrepancies. A comparative analysis of setup accuracy was performed between the two positioning methodologies. Additionally, margin expansion formulas were applied to assess the optimization potential of clinical target volume-to-planning target volume (CTV-PTV) margins.

Results: In the DIBH subgroup, the OSMS group exhibited significantly lower translational errors along the X- and Z-axes compared to the conventional group (0.22 ± 0.16 cm vs. 0.33 ± 0.27 cm; 0.20 ± 0.18 cm vs. 0.42 ± 0.32 cm, respectively; *P* < 0.05), while no significant differences were observed in rotational errors. Within the FB subgroup, no statistically significant differences were found between the two groups. For DIBH patients treated with OSMS, the CTV-PTV margin expansion could be optimized to 4.2–5.1 mm, and a standardized 5-mm margin protocol is recommended for clinical implementation to ensure consistency.

Conclusion: This study demonstrates that the application of the OSMS automatic positioning function significantly reduces the expanding clinical target volume (CTV) for breast cancer patients with breast bracket fixation and the DIBH technique during radiotherapy. The required expanding margins for CTV-PTV in the x, y, and z directions are 4.2 mm, 5.1 mm, and 4.8 mm, respectively. Due to residual errors from respiratory motion and clinical practices, it is recommended to adopt an expansion plan of 5 mm.