Timing of computed tomography-based postimplant assessment following permanent transperineal prostate brachytherapy

To establish the rate of resolution of prostatic edema following transperineal interstitial permanent prostate brachytherapy, and to determine the results and impact of timing of the postimplant assessment on the dose-volume relationship.

A series of 19 consecutive patients with early-stage adenocarcinoma of the prostate receiving transperineal interstitial permanent prostate brachytherapy, were enrolled in this study. Twelve received 125I and seven received 103Pd. Postoperative assessment included a computed tomographic (CT) scan on postoperative days 1, 8, 30, 90, and 180. On each occasion, CT scans were performed on a GE helical unit at 3-mm abutting slices, 15-cm field of view. Prostate volumes were outlined on CT scans by a single clinician. Following digitization of the volumes and radioactive sources, volumes and dose-volume histograms were calculated. The prostate volume encompassed by the 80% and 100% reference isodose volumes was calculated.

Preimplant transrectal ultrasound determined volumes varied from 17.5 to 38.6 cc (median 27.9 cc). Prostate volumes previously defined on 40 randomly selected postimplant CT scans were compared in a blinded fashion to a second CT-derived volume and ranged from -32% to +24%. The Pearson correlation coefficient for prostate CT volume reproducibility was 0.77 (p < 0.03). CT scan-determined volume performed on postoperative day 1 was an average of 41.4% greater than the volume determined by preimplant ultrasound. Significant decreases in average volume were seen during the first month postoperatively. Average volume decreased 14% from day 1 to day 8, 10% from day 8 to day 30, 3% from day 30 to day 90, and 2% thereafter. Coverage of the prostate volume by the 80% isodose volume increased from 85.6% on postoperative day 1 to 92.2% on postoperative day 180. The corresponding increase in the 100% reference dose coverage of the prostate volume ranged from 73.1% to 83.3% between postoperative days 1, and 180, respectively.

Most of the prostatic edema induced by brachytherapy appears to resolve by postoperative day 30. Scans performed on postimplant day 30 appear to adequately describe the time-averaged dose coverage of the prostate. This suggests that waiting approximately 1 month to perform postimplant analysis gives the most accurate prostatic volume and, consequently, dosimetric description of the implant.