Postimplantation dosimetric analysis of permanent transperineal prostate implantation: Improved dose distributions with an intra operative computer-optimized conformal planning technique

Purpose: To compare the target coverage and dose to normal tissues after I- 125 transperineal permanent implantation (TPI) of the prostate in 90 patien ts treated with one of three different transperineal techniques. Methods and Materials: Detailed postimplant dosimetric evaluations of perma nent I-125 implantation procedures were performed on 30 consecutive patient s treated between 1995-1996 who underwent TPI using a preplanning CT-based technique, on 30 consecutive patients treated in 1997-1998 who underwent an ultrasound-guided approach with intraoperative determination of seed distr ibution based on an I-125 nomogram, and on 30 consecutive patients in 1998- 1999 who underwent TPI with intraoperative computer-based 3-dimensional con formal optimization. For all three techniques, postimplant CT scans were ob tained 4-6 hours after TPI, Dosimetric parameters included V-100, V-90, V-1 50, D-100, D-90, D-80, as well as maximal and average doses to the urethra and rectal wall. These parameter outcomes are reported as a percentage of t he prescription dose. Results: The intraoperative 3D-optimized technique (I-3D) provided superior target coverage with the prescription dose for all dosimetric variables ev aluated compared to the other treatment techniques. The median V-100, V-90, and D-90 values for the I-3D technique were 96%, 98%, and 116%, respective ly. In contrast, the V-100, V-90, and D-90 values for the CT preplan and ul trasound manual optimization approaches were 86%, 89%, and 88%, respectivel y and 88%, 92%, and 94%, respectively (I-3D versus other techniques: p < 0. 001). The superior target coverage with the I-3D technique was also associa ted with a higher cumulative implant activity required by the optimization program. A multivariate analysis determined that the treatment technique (I -3D versus other approaches) was an independent predictor of improved targe t coverage for each parameter analyzed (p < 0.001). in addition, higher cum ulative implant activities and smaller prostate target volumes were indepen dent predictors of improved target coverage, The maximum and average urethr al doses were significantly lower with the I-3D technique compared to the o ther techniques; a modest increase in the average rectal dose was also obse rved with this approach, Conclusion: Three-dimensional intraoperative computer optimized TPI consist ently provided superior target coverage with the prescription dose and sign ificantly lower urethral doses compared to two other techniques used. These data provide proof-of-principle that improved therapeutic ratios can be ac hieved with the integration of more sophisticated intraoperative planning f or TPI and may potentially have a profound impact on the outcome of patient s treated with this modality. (C) 2000 Elsevier Science Inc.