Dosimetric consequences of using a surrogate urethra to estimate urethral dose after brachytherapy for prostate cancer.

PURPOSE: To assess the accuracy and dosimetric consequences of defining a surrogate urethra at the geometric center of the prostate in postimplant CT scans. METHODS AND MATERIALS: Eighty postimplant CT scans were obtained with a Foley catheter in place at Day 0 and at 1 month for 40 patients who had undergone (125)I prostate brachytherapy. The percentage of urethral volume receiving at least 275% of the prescribed dose (uV(275)), uV(250), uV(200), uV(150), maximal dose received by 90% of urethral volume (uD(90)), uD(70), uD(30), and uD(1) were measured for the Foley catheter and surrogate urethra. The distance between the Foley catheter and surrogate urethra was measured at the base, middle, and apex of the prostate. RESULTS: A statistically significant difference was found in all the above-listed dosimetric parameters between the Foley catheter and surrogate urethra at Day 0 (p