Comparison of urethral diameters for calculating the urethral dose after permanent prostate brachytherapy

Purpose. No studies have yet evaluated the effects of a dosimetric analysis for different urethral volumes. We therefore evaluated the effects of a dosimetric analysis to determine the different urethral volumes. Methods. This study was based on computed tomography/magnetic resonance imaging (CT/MRI) combined findings in 30 patients who had undergone prostate brachytherapy. Postimplant CT/MRI scans were performed 30 days after the implant. The urethra was contoured based on its diameter (8, 6, 4, 2, and 0 mm). The total urethral volume-in cubic centimeters [UrV150/200(cc)] and percent (UrV150%/200%), of the urethra receiving 150% or 200% of the prescribed dose-and the doses (UrD90/30/5) in Grays to 90%, 30%, and 5% of the urethral volume were measured based on the urethral diameters. Results. The UrV150(cc) and UrD30 were statistically different between the of 8-, 6-, 4-, 2-, and 0-mm diameters, whereas the UrD5 was statistically different only between the 8-, 6-, and 4-mm diameters. Especially for UrD5, there was an approximately 40-Gy difference between the mean values for the 8- and 0-mm diameters. Conclusion. We recommend that the urethra should be contoured as a 4- to 6-mm diameter circle or one side of a triangle of 5-7 mm. By standardizing the urethral diameter, the urethral dose will be less affected by the total urethral volume.