The impact of postimplant edema on the urethral dose in prostate brachytherapy

Purpose: The objective of this work is to determine the effect of timing of the postimplant CT scan on the assessment of the urethral dose. Methods and Materials: A preimplant CT scan and two postimplant CT scans we re obtained on 50 patients who received I-125 prostate seed implants. The f irst postimplant CT scan was obtained on the day of the implant; the second usually 4 to 9 weeks later (mean: 46 +/- 23 days; range: 27-135 days). The urethra was localized in each postimplant CT scan and a dose-volume histog ram (DVH) of the urethral dose was compiled from each CT study. The relativ e decrease in the prostate volume between the first and second postimplant CT scans was determined by contouring the prostate in each CT scan. Results: The prostate volume decreased by 27 +/- 9% (mean +/- SD) between t he first and second postimplant CT scans. As a result, the averaged urethra l dose derived from the second CT scan was about 30% higher. In terms of do se, the D-10, D-25, D-50, D-75, and D-90 urethral doses derived from the se cond CT scan were 90 +/- 56 Gy, 81 +/- 49 Gy, 67 +/- 42 Gy, 49 +/- 44 Gy, a nd 40 +/- 46 Gy higher, respectively. The increase in the urethral dose is correlated with the decrease in the prostate volume (R = 0.57, rho < 0.01). Conclusion: The assessment of the urethral dose depends upon the timing of the postimplant CT scan. The mean D-10 dose derived from the CT scans obtai ned at 46 +/- 23 days postimplant was 90 +/- 56 Gy higher than that derived from the CT scans obtained on the day of the implant. Because of this larg e difference, the timing of the postimplant CT scan needs to be specified w hen specifying dose thresholds for urethral morbidity. (C) 2000 Elsevier Sc ience Inc.