Poor Predictive Value of Intraoperative Real-Time Dosimetry for Prostate Seed Brachytherapy

Purpose To identify dosimetric parameters predictive of a good prostate seed I125 quality implant. We analyzed preimplant and postimplant realtime dosimetry in patients treated with intraoperative (IO) inverse planning. Methods and Materials We analyzed 127 consecutively treated patients with primarily low-risk prostate carcinoma who underwent prostate permanent seed I125 brachytherapy using an IO planning approach. The implant was done using the three-dimensional transrectal ultrasound (PRE-TRUS)-guided IO interactive inverse preplanning system. The TRUS was repeated in the operating room after the implant procedure was complete (POST-TRUS). The prostate was recontoured and postimplant dosimetry was calculated. Each patient underwent computed tomography scan on Day 28 (CT-D28) to evaluate implant quality. Area under the receiver operating characteristic curves (AUROC) was evaluated for models predictive of a V100 of ≥90% and a D90 of ≥140 Gy on the basis of CT-D28 values. Results On CT-D28, 72.4% of patients had a V100 of ≥90% and 74.8% had a D90 of ≥140 Gy. AUROC for a V100 of ≥90% was 0.665 ( p = 0.004) on PRE-TRUS and 0.619 ( p = 0.039) on POST-TRUS. AUROC for D90 of ≥140 Gy was 0.602 ( p = 0.086) on PRE-TRUS and 0.614 ( p = 0.054) on POST-TRUS. Using PRE-TRUS V100 cutoff of >97% gives sensitivity of 88% and a false-positive rate of 63%. A POST-TRUS D90 cutoff of >170 Gy resulted in a sensitivity of 62% and a false-positive rate of 34%. Conclusions Because of unacceptably high false-positive rates, IO preimplant and postimplant TRUS-based dosimetry are not accurate tools to predict for postimplant computed tomography–based dosimetry.