The importance of urethra visualization for preplanned permanent prostate implants

Purpose To assess the potential consequences of using a surrogate urethra on urethral dose estimates in preplanned 125I prostate implants. Methods and materials For n = 220 patients, the A-P and L-R extents of prostate and urethra contours were measured in transrectal ultrasound images. Treatment plans were then developed for 6 patients, of which 5 had atypical urethral positions. For each patient, three plan variations were made using the visualized and two different surrogate urethra contours. Results The urethra typically remains fixed in the L–R direction and extends slightly below midgland, but may veer off-center and can come within 0.5 cm of the posterior surface of the prostate. Use of a surrogate urethra can potentially result in up to 30% of the urethra receiving doses exceeding a planned limit of 1.5 × 145 Gy over a contiguous length of 2.0 cm. Conclusions The urethra should be visualized for preplanning purposes, because unintended urethral doses arising from the use of a surrogate urethra can approach levels associated with late urinary morbidity. Visualization is also essential in the postimplant setting for accurate collection of dose–toxicity data.