Comparison of intraoperative dosimetric implant representation with postimplant dosimetry in patients receiving prostate brachytherapy

Purpose: To compare the results of intraoperative dosimetry with those of CT-based postimplant dosimetry in patients undergoing prostate seed implantation. Methods and Materials: Seventy-seven patients with T1–T3 prostate cancer received an ultrasound-guided permanent seed implant (36 received 125I, 7 103Pd, and 34 a partial 103Pd implant plus external beam radiation therapy). The implantation was augmented with an intraoperative dosimetric planning system. After the peripheral needles were placed, 5-mm axial images were acquired into the treatment planning system. Soft tissue structures (prostate, urethra, and rectum) were contoured, and exact needle positions were registered. Seeds were placed with an applicator, and their positions were entered into the planning system. The dose distributions for the implant were calculated after interior needle and seed placement. Postimplant dosimetry was performed 1 month later on the basis of CT imaging. Prostate and urethral doses were compared, by using paired t tests, for the real-time dosimetry in the operating room (OR) and the postimplant dosimetry. Results: The mean preimplant prostate volume was 39.8 cm3, the postneedle planning volume was 41.5 cm3 (p<0.001), and the 1-month CT volume was 43.6 cm3 (p<0.001). The mean difference between the OR dose received by 90% of the prostate (D90) and the CT D90 was 3.4% (95% confidence interval, 2.5–6.6%; p = 0.034). The mean dose to 30% of the urethra was 120% of prescription in the OR and 138% on CT. The mean difference was 18% (95% confidence interval, 13–24%; p<0.001). Conclusions: Although small differences exist between the OR and CT dosimetry results, these data suggest that this intraoperative implant dosimetric representation system provides a close match to the actual delivered doses. These data support the use of this system to modify the implant during surgery to achieve more consistent dosimetry results.