Determination of the urethral dose in prostate brachytherapy when the urethra cannot be visualized in the postimplant CT scan

This study investigates the feasibility of locating the urethra at the geometric center of peripherally loaded 125 I prostate implant when a urinary catheter is not utilized for the postimplant CT scan. Twenty postimplant CT scans utilizing a urinary catheter were randomly selected. The urethra was localized in each study and, in addition, a surrogate urethra was localized at the geometric center of the prostate. Dose-volume histograms of the urethra and surrogate urethra were compiled and compared. The values obtained for the urethra D 10 , D 25 , and D 50 were in good agreement and demonstrate that the urethral dose can be determined reliably by locating a surrogate urethra at the geometric center of the prostate in a peripherally loaded implant when the urethra cannot be visualized.