Rectal content and intrafractional prostate gland motion assessed by magnetic resonance imaging.

We evaluated the interrelationship between rectal content and intrafraction motion of the prostate. Forty seven prostate cancer patients instructed to remove their rectal gas were imaged by planning CT and MRI before radiotherapy (RT) and during RT. The total scan time was comparable to our cone-beam CT scanning and treatment times. Rectal content was qualitatively assessed into four different categories by T2-weighted axial MRI: empty (Group E), gas (Group G), combination of gas and feces (Group C), and feces (Group F). Eleven anatomic points of interest (POI) were determined on subsequent sagittal cine-MRI slices. The incidence of displacement of more than 3 mm for more than 10% of time (> 10% time over 3 mm) at least in one of the prostate POIs in Group E was 6.3%, Group G 40.9%, Group C 6.3%, and Group F 0%, respectively. Except for Group G, the mean probability of > 3 mm displacement was < 3%. More than 10% time over 3 mm displacement of the superior prostate in the AP direction (SAP) was noted in only Group G patients and was 45.5% before RT and 18.2% during RT. Only Group G patients were significantly related to both the mean of means and the mean of maxs of prostate displacement of SAP by multivariate analysis. Group G patients were also significantly related to the mean of the standard deviation of rectum width of superior rectum and mid-rectum by multivariate analysis. Patients with rectal gas only were significantly related to prostate displacement and rectal movement.