Abstract
To assess the accuracy and dosimetric consequences of defining a surrogate urethra at the geometric center of the prostate in postimplant CT scans.
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
125I 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.
A statistically significant difference was found in all the above-listed dosimetric parameters between the Foley catheter and surrogate urethra at Day 0 (
p ≤ 0.001). At 1 month, the uD
90, uD
70, and uD
1 remained significantly different between the Foley catheter and surrogate urethra (
p ≤ 0.05). The difference in the uV
275 (
p = 0.055) and uV
150 (
p = 0.059) between the Foley catheter and surrogate urethra showed a trend toward statistical significance at 1 month. The uV
250, uV
200, and uD
30 were greater for the surrogate urethra than for the Foley catheter at 1 month, but were not significantly different statistically. The mean distance between the Foley catheter and the surrogate urethra was greatest at the base (1.2 cm) in the vertical axis at Day 0 and decreased substantially to 0.87 cm at 1 month (
p = 0.0004).
Using a surrogate urethra at the geometric center of the prostate may significantly overestimate the urethral dose at Day 0 and certain dosimetric parameters at 1 month. An alternative position for a surrogate urethra accounting for the difference in the location of the Foley catheter near the base of the prostate at Day 0 and 1 month could be considered in future studies.
