2865 - Intra-Fractional Bladder Volume Dynamics and Stable Rectal Geometry during Postoperative Radiotherapy for Endometrial Cancer with CT-Linac: Implications for Margin Optimization
Presenter(s)
L. Wan1, Y. Jiang1, R. Lei2, W. Zhao1, and A. Qu2; 1School of Physics, Beihang University, Beijing, China, 2Department of Radiation Oncology, Peking University Third Hospital, Beijing, China
Purpose/Objective(s): This study aims to investigate intra-fractional bladder and rectal volume changes in patients undergoing postoperative radiotherapy for endometrial cancer (EC) and to provide a reference for the development of an adaptive radiotherapy (ART) plan.
Materials/Methods: This study included 14 postoperative EC patients who underwent radiotherapy using an integrated CT-Linac system (uRT-linac 506c, United-Imaging). Each patient underwent three fan beam CT (FBCT) scans per treatment fraction: before positioning (FBCT 1st), after positioning (FBCT 2nd), and post-treatment (FBCT 3rd). For bladder analysis, 247 treatment fractions (741 FBCT sets) were assessed, while rectum analysis included 319 treatment fractions (957 FBCT sets). A deep learning model was employed for automatic segmentation of the bladder and rectum, with senior physicists reviewing and refining the contours to ensure high accuracy. The segmented bladder and rectum volumes were quantified and compared across scans.
Results: In bladder volume analysis, the mean volumes for FBCT 1st, FBCT 2nd, and FBCT 3rd were 279.56 ± 133.15 cc, 292.13 ± 135.36 cc, and 337.41 ± 140.88 cc, respectively, indicating a consistent volume increase throughout the scans. Based on the bladder segmentation from the FBCT 1st scan, the volume distribution was categorized into six groups: <100 cc, 100-200 cc, 200-300 cc, 300-400 cc, 400-500 cc, and >500 cc, with 17, 68, 67, 46, 32, and 10 fractions in each group, respectively. The percentage volume increase from FBCT 2nd to FBCT 3rd in each group was 30%, 18.78%, 13.12%, 13.22%, 9.33%, and 6.21%, respectively. For rectum volume analysis, the mean volumes of FBCT 1st, FBCT 2nd, and FBCT 3rd were 54.49 ± 16.40 cc, 54.23 ± 17.09 cc, and 55.19 ± 16.73 cc, respectively, showing minimal variation across scans. The time intervals between FBCT 1st and FBCT 2nd, and between FBCT 2nd and FBCT 3rd, were 2.09 ± 0.91 minutes and 7.08 ± 1.33 minutes, respectively.
Conclusion: During the several minutes of radiotherapy treatment, rectum volume remains relatively stable, while bladder volume undergoes significant changes, with the extent of fluctuation being inversely correlated with bladder filling. It is recommended that physicians set an appropriate expansion margin based on the patient's bladder filling status when formulating the online ART plan. These findings provide actionable data for optimizing the efficiency and precision of adaptive planning in postoperative radiotherapy for EC.