MRI-CT fusion to assess postbrachytherapy prostate volume and the effects of prolonged edema on dosimetry following transperineal interstitial permanent prostate brachytherapy.

Purpose: Quality assurance through postplan assessment is an integral part of permanent seed prostate implants. The use of MRI-CT fusion for 1-month postimplant dosimetry permits accurate assessment of prostate volume without seed induced artifact and uncertainties of prostate contour inherent to CT assessments. Routine use of MRI-CT fusion reveals significant prostate edema may persist several weeks. This study evaluates the effect of edema, and its subsequent resolution, on dosimetry.

Methods And Materials: From May 2001 to June 2003, 241 men were treated with (125)I seed implants based on a transrectal ultrasound (TRUS) preplan. Quality assessment was performed at 1 month by CT-MRI fusion using VariSeed software. Over this 24-month period, 29 patients (12%) with residual edema at 1 month (12-60% >TRUS plan volume), had repeat CT-MRI fusion at 2-4 months to reassess volume and dosimetry. Eleven of the 29 had received prior androgen ablation to shrink the prostate preimplant.

Results: For the entire group (n = 241), mean preimplant prostate volume was 33.7 cc and median postplan dosimetric parameters were: V100, 92.2%; D90, 153 Gy; and V150, 53%. For the 29 patients with prolonged edema, mean preimplant volume was 34.8 cc and 1-month volume was 46.1 cc (p <0.001). Mean volume reduction between 1 and 2 months was 13%. The decrease in prostate volume had a significant effect on dosimetry with median increase between 1 month and 2 months in calculated V100 of 9.5%, V150 22.6%, V200 30.1%, and D90 11.5%.

Conclusions: Significant residual edema is seen 1-month postimplant in 12% of prostates and may have a profound effect on dosimetry. Further study is underway to characterize the time course of resolution of the edema, and to perform integral dosimetry based on the changing volume.