Assessing intra-fractional bladder motion using cine-MRI as initial methodology for Predictive Organ Localization (POLO) in radiotherapy for bladder cancer

AimTo assess the feasibility of using cine-MR to study intra-fractional time–volume and volume-deformity patterns of the bladder during radiotherapy as initial methodology for Predictive Organ Localization (POLO).MethodsNine patients receiving radiotherapy for localized muscle invasive bladder cancer were prospectively studied. Each had an MR scan performed on an empty bladder using a T1 weighted cine sequence over a period of 20 min. Scans were taken prior to, and repeated towards the end of, radiotherapy treatment. Time–volume sequences were determined and compared before and during radiotherapy. Absolute bladder volumes were then correlated with changes in bladder wall position.ResultsThe mean post void residual bladder volume prior to radiotherapy at time 0 was 113 cm3 [SD 53] and this did not differ significantly during radiotherapy −106 cm [SD 40] (p = 0.24, paired t-test analysis). A linear relationship was observed for the rate bladder filling over a 20 min period, which did not significantly change on the cine-MR during radiotherapy (regression coefficient 2.1 vs 1.6, respectively, p = 0.51). Significant positive relationships were seen between volume and anterior (p = 0.02), superior (p < 0.001), and inferior (p = 0.03) wall motion. These relationships were complex, though linearity was observed for volumes up to 150 cm3. The 1.5 cm CTV–PTV margin was sufficient to account for expansion in the majority of cases with the only breach occurring on the anterior wall in one patient.ConclusionsThis study confirms the feasibility of using cine-MR for POLO. The development of such predictive methodology may compensate for the need to use an isotropic CTV–PTV margin to simply cover bladder filling when using image-guided radiotherapy.