ESTRO 2025 - Abstract Book

S3328

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2025

3745

Digital Poster Refining bladder filling strategies in MR-guided adaptive radiotherapy for rectal cancer

Min Liu, Ke Yuan, Bin Tang, Yilin Wang, Xi Feng, Xin Xin, Jie Li, Xianliang Wang, Na Huang, Lucia Clara Orlandini Department of Radiation Oncology, Sichuan Cancer Hospital & Institute, Affiliated Cancer Hospital of University and Electronic Science and Technology of China, Chengdu, China Purpose/Objective: The treatment of rectal cancer with MR-guided adaptive radiotherapy is time-intensive, making organ-at-risk deformation, particularly bladder management, a critical challenge. A previous study developed a procedure to minimize bladder variability, but it was relatively invasive. This study aims to develop a more patient-friendly approach maintaining efficacy and improving tolerability. Material/Methods: Data from 80 online adaptive radiotherapy fractions were analyzed for 16 rectal cancer patients undergoing short course radiotherapy (25 Gy, 5Gy/fr x 5) using the Unity 1.5 T MR-Linac. To regulate bladder filling, patients limited fluid intake for two hours before treatment, and a bladder catheter was used. At the start of each session, saline was instilled for controlled filling, and an initial MRI was acquired to adapt the treatment plan for targets and OARs. After plan approval, a second MRI re-delineated the bladder, measured its volume, and compared it to the first MRI. Excess fluid was drained to ensure consistency. A third MRI confirmed bladder volume and shape consistency at session end. Unlike the previous method requiring bladder emptying and refilling before plan adaptation and beam delivery, this approach involves only initial filling and minor drainage, reducing invasiveness. Figure 1 illustrates the bladder volume adjustments during the online ART process. Follow-up assessments monitored urinary tract infection incidence related to catheter use. Changes in the bladder volumes between the adapted plan and beam delivery were analyzed using the Dice Similarity Index (DSC) and Hausdorff Distance (HD). Statistical analyses were conducted using SPSS software with paired t-tests or Wilcoxon signed-rank tests.

Results: The average changes in bladder volume, DSC, and HD between the first and third MRI were 6.14%, 0.938, and 2.54 mm, respectively, with a median time interval of 41 minutes. Differences in target prescription dose coverage were below 0.53%, while dose variations for the bladder, small intestine, and femoral heads were within 0.5%. Table 1 provides a detailed summary. Among 16 patients, 15 had catheter placement for less than five days, with no urinary tract infections. Six experienced mild hematuria during later treatment stages, resolving with increased fluid intake. Two patients with prostate hyperplasia reported urinary discomfort, including frequency and urgency, improving with tamsulosin. One patient required catheter placement for over 10 days and developed a urinary tract infection, resolving with two weeks of levofloxacin.