To move or not to move: Measurements of prostate motion by urethrography using MRI

Purpose: Urethrography is commonly used to aid in definition of the prostate apex during CT simulation for prostate cancer. If the position of the prostate were altered by the urethrogram itself, then systematic error could be introduced into the patient's treatment. Sagittal MRI scans were acquired immediately before and after a localization urethrogram to determine the extent of displacement. Methods and Materials: Thirteen patients underwent sagittal T2-weighted fast spin echo MRI scans. Patients were scanned supine in an alpha cradle cast in the treatment position. The prostate was contoured by 3 different observers to determine the apex location on the central sagittal MRI section and the center of mass relative to an immobile bony landmark. Statistical multivariate analysis was performed to establish if there was a net displacement of the prostate (systematic error), and to determine the margin required to cover the random prostate position within a 95% confidence interval. Results: There was no significant systematic motion of either the prostate nor its apex in either the anterior-posterior or superior-inferior directions. The average motion of the prostate center of mass was 0.04 ± 0.40 cm (1 SD) and 0.01 ± 0.33 cm in the anterior-posterior and superior-inferior direction, respectively. The corresponding figures for location of the apex were 0.05 ± 0.30 cm and 0.01 ± 0.33 cm, respectively. The statistical analysis revealed that a margin of 2 mm is sufficient to cover any random motion of the prostate that could occur as a result of the urethrogram 95% of the time. Conclusion: Urethrography during CT simulation for prostate cancer does not cause significant prostate displacement or systematic error in planning and delivering external-beam radiation.