ESTRO 2021 Abstract Book

S1657

ESTRO 2021

Conclusion SGRT showed to be a promising technique regarding accuracy of patient positioning. It makes possible to reduce the thermoplastic masks and skin markers usage without compromising positioning accuracy.

PO-1945 SGRT Positioning Accuracy and Consistency in Mastectomy Patients in terms of ROI and Immobilization J. Li 1 , S. Yu 1 , Y. Du 1 , S. Zhou 1 , H. Yue 1 , M. Wang 1 , J. Yang 1 , H. Wu 1 , T. Li 2 1 Peking University Cancer Hospital & Institute, Department of Radiation Oncology, Beijing, China; 2 The Fifth Medical Center of PLA General Hospital, Department of Radiation Oncology, Beijing, China Purpose or Objective This study aims to investigate the surface-guided radiotherapy (SGRT) positioning accuracy and consistency on mastectomy patients by suggesting an optimal region of interest (ROI) and immobilization approach. Materials and Methods 6 mastectomy patients with vac-bag and 12 mastectomy patients with neck and thorax thermoplastic were enrolled. Three ROIs were drawn for each patient respectively: bilateral ROI (ROI A), ipsilateral ROI (ROI B) and ipsilateral with rigid contralateral ROI (ROI C). All patients were aligned using SGRT system based on ROI A, and then switched to ROI B and C. All shifts shown on SGRT were recorded and followed by CBCT acquisition. The CBCT-to-planning CT registration was first performed by spine alignment and then minorly adjusted to achieve PTV matching. The six degree-of-freedom (6DoF) offset discrepancies from AlignRT to CBCT were analyzed using Friedman M test and Mann-Whiteney U test to statistically investigate the influence of ROI contouring and immobilization approach. Results A total of 110 CBCT scans were analyzed. As in Table 1 , bilateral ROI exhibits better positioning accuracy and consistency in PITCH (p = 0) and ROLL (p = 0.025). As in Table 2 , while the difference in positioning accuracy between vac-bag and thermoplastic was generally insignificant, vac-bag exhibits superior positioning consistency in VRT (p = 0) and RTN (p = 0.041).

Conclusion Bilateral ROI and vac-bag are found to be the optimal positioning solution for SGRT mastectomy patients with improved positioning accuracy and consistency.

Digital Poster: Imaging acquisition and registration, OAR and target definition