Purpose: To evaluate the dynamic interrelationship between rectal distension and rectal movements, and to determine the effect of rectal movement on the position of the prostatic gland using cine magnetic resonance imaging (MRI).
Methods and materials: Fifty-five patients with biopsy-proven or suspected prostate cancer were examined in the axial plane using repeated spoiled gradient-echo sequences every 10 seconds for 7 minutes. Twenty-four patients received bowel relaxants before imaging. Images were analyzed for the degree of rectal distension, for the incidence, magnitude, and number of rectal and prostate movements.
Results: Rectal movements were seen in 28 (51%) patients overall, in 10 (42%) of those receiving bowel relaxants and in 18 (58%) not receiving bowel relaxants. The incidence of rectal movements correlated with the degree of rectal distension (p = 0.0005), but the magnitude of rectal movements did not correlate with the degree of rectal distension. Eighty-six rectal movements resulting in 33 anterior-posterior (AP) prostate movements were seen. The magnitude of rectal movements correlated well with degree of prostate movements (p < 0.001). Prostate movements in the AP direction were seen in 16 (29%) patients, and in 9 (16%) patients the movement was greater than 5 mm. The median prostate AP displacement was anterior by 4.2 (-5 to +14 mm).
Conclusions: Cine MRI is able to demonstrate near real time rectal and associated prostate movements. Rectal movements are related to rectal distension and result in significant displacements of the prostate gland over a time period similar to that used for daily fractionated radiotherapy treatments. Delivery of radiotherapy needs to take into account these organ movements.