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 were obtained on 50 patients who received I-125 prostate seed implants. The first 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 histogram (DVH) of the urethral dose was compiled from each CT study. The relative 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 the first and second postimplant CT scans. As a result, the averaged urethral dose derived from the second CT scan was about 30% higher. In terms of dose, the D(10), D(25), D(50), D(75), and D(90) urethral doses derived from the second CT scan were 90 +/- 56 Gy, 81 +/- 49 Gy, 67 +/- 42 Gy, 49 +/- 44 Gy, and 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 obtained 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 large difference, the timing of the postimplant CT scan needs to be specified when specifying dose thresholds for urethral morbidity.