Inter- and Intrafraction Bladder and Rectum Motion in Patients With Cervical Cancer Under MR-Guided Radiotherapy on a 1.5T MR-Linac

Internal bladder and rectum motion occur during radiotherapy for cervix cancer. IMRT planning studies have shown that the quality of radiation delivery is influenced by these anatomical changes. The MR-Linac system allows for daily MR image guidance and real-time imaging throughout the treatment fraction, which is ideal for managing and monitoring both inter-fraction and intrafraction motion, without incurring additional radiation exposure. The study is to evaluate bladder and rectum inter- and intrafraction motion using MRI during the full course of MR-Linac radiotherapy fractions, in preparation of MR-guided online adaptive radiotherapy.Five stage ⅡA - ⅢB cervix cancer patients were treated with 25 × 1.8 Gy fractions on a 1.5T MR-Linac. Each fraction, pre-treatment MRI scans were obtained at the start of every treatment session, and post-treatment MRI scans were obtained at the end of every treatment session. The bladder and rectum were delineated on each MRI by the same radiation oncologist. Inter-fraction and intrafraction variability were evaluated with the Dice similarity coefficient (DSC), the Hausdorff distance (HD), mean distance to agreement (MDA), volume (V) and dose volume parameters.Large inter-fraction motion was found for bladder and rectum due to their filling variation (Bladder: DSC: 0.68 ± 0.17, HD: 30.58 ± 15.32, MDA: 7.77 ± 4.65, V: -23.02% ± 13.13%, Dmean: 2.23% ± 7.10; Rectum: DSC: 0.69 ± 0.04, HD: 22.98 ± 4.02, MDA: 3.45 ± 0.63, V: 13.57% ± 18.09%, Dmean: 0.4% ± 0.7%). Median in-room treatment time pre fraction was 35 min which was much longer than conventional treatment time. The intrafraction motion for bladder was obvious that was explained by the bladder filling variation due to prolonged in-room treatment time per fraction (DSC: 0.88 ± 0.07, HD: 14.66 ± 8.48, MDA: 1.89 ± 1.15, V: 13.06% ± 15.66%, Dmean: -0.89% ± 2.45%). The intrafraction rectum motion was much smaller.Considerable inter-fraction bladder and rectum motion is observed and intrafraction bladder motion which caused by longer treatment time is also obvious in cervical cancer patients on 1.5T MR-Linac. Intrafraction MR-guided online adaptive radiotherapy may potentially further correct this motion.S. Ding: None. H. Liu: None. B. Wang: None. Y. Li: None. B. Liu: None. Y. Ouyang: None. X. Huang: None.