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 prostat e apex during CT simulation for prostate cancer. If the position of the pro state were altered by the urethrogram itself, then systematic error could b e introduced into the patient's treatment. Sagittal MRI scans were acquired immediately before and after a localization urethrogram to determine the e xtent of displacement. Methods and Materials: Thirteen patients underwent sagittal T2-weighted fas t spin echo MRI scans. Patients were scanned supine in an alpha cradle cast in the treatment position. The prostate was contoured by 3 different obser vers to determine the apex location on the central sagittal MRI section and the center of mass relative to an immobile bony landmark. Statistical mult ivariate analysis was performed to establish if there was a net displacemen t 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 directio ns. 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 ape x 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 an d delivering external-beam radiation. (C) 2001 Elsevier Science Inc.