The importance of urethra visualization for preplanned permanent prostate implants

To assess the potential consequences of using a surrogate urethra on urethral dose estimates in preplanned 125I prostate implants.

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.

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 x 145 Gy over a contiguous length of 2.0 cm.

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.