Time-dependent changes in CT-based dosimetry of I-125 prostate brachytherapy

To determine the effect of time between prostate brachytherapy and evaluation CT scan on calculated target coverage. CT scans from 11 consecutive, unselected patients with stage T1 or T2 prostatic carcinoma who had transperineal I-125 implants at MSKCC in 1996 were analyzed for target coverage at 0, 2, and 6 months after implantation. The outer margins of the prostate were outlined on each CT section by a single investigator. In each CT plane, the prescription isodose (150 Gy) was overlaid on the target contour from the postimplant CT to calculate the integral Dose-Volume Histogram. The postimplant target volume on the day of the implant ranged from 93% to 160% of the preimplant volume (average: 117%). In all patients, the target size returned to the preimplant size or smaller within 2 months of the procedure and was relatively stable between 2 to 6 months. Immediately following the implant, an average of 84% of the target (range: 73-98%) was covered by the 150 Gy isodose line. Consistent with changes in the target volume over time, the target coverage increased from an average of 84% to 90% between 0 to 2 months and did not change substantially between 2 and 6 months. There was minimal source loss from the target area after the implant. It was concluded that temporary, postimplant swelling will increase the target volume, making target coverage inferior to what would be calculated if a dosimetry scan was taken sometime later, after the acute swelling has subsided. Until the clinical significance of the effect of postimplant volume changes is better defined, we are continuing to obtain evaluation scans on the day of the implant.