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

AIM: To 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).
METHODS: Nine 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.
RESULTS: The mean post void residual bladder volume prior to radiotherapy at time 0 was 113 cm(3) [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 cm(3). 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.
CONCLUSIONS: This 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.