Intraoperative Prostate Volume Change During Permanent Low-Dose-Rate I-125 Prostate Brachytherapy: Implications for Final Implant Dosimetry

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To evaluate the magnitude of real-time intra-operative change in prostatic volume during low-dose-rate I-125 prostate brachytherapy, to determine dosimetric implications to the target of this volume change, and to evaluate the associations between pre-procedural prostate volume, real-time intraoperative prostate volume change and day 30 prostate volume change. The prostate was retrospectively contoured on the pre-procedural and day 30 CT scans as well as on the intraoperative real-time pre- and postoperative transrectal ultrasound generated images of 37 patients treated with low-dose-rate I-125 permanent prostatic seed implant at UC Davis Medical Center. Real-time TRUS images were obtained just prior to initiation of the implant, and immediately following placement of the final seed. The average elapsed time between the pre- and postoperative TRUS image was 124 minutes. For TRUS images, SPOT-PRO was used to delineate pre- and post-procedural target volumes using serial 2.5 mm axial images. For CT images, Pinnacle was used to generate pre- and day 30 post-procedural target volumes using serial 3mm axial images. The resultant dosimetry based on these volumes was compared. Pre-procedural CT derived prostate volumes were evaluated as prognosticator of intraoperative volume change. Intraoperative volume change was evaluated as a prognosticator of Day 30 volume change. The volume of the prostate increased by an average of 22% between the time of the initial and final real-time intraoperative TRUS image acquisitions. There was a wide range of volume change (8-46%). No significant correlation between pre-procedural CT or TRUS derived prostate volume and the magnitude of intraoperative TRUS derived volume change was noted. There was a non-significant correlation between the magnitude of intraoperative volume change and the magnitude of the CT derived day 30 prostate volume change. The absolute volume change was comparable between smaller and larger prostate glands, but the resultant change in dosimetry attributable to the volume change was more significant for smaller prostate glands. For patients with smaller prostate glands (<35cc) exhibiting higher than average volume change on post-implant intraoperative real-time TRUS, dose to the target (D90, V100) may be lower than desired. The impact of this dose reduction on clinical outcome is not known, but may merit consideration when tumor is known to involve those areas most prone to volume expansion. Neither CT derived pre-procedural nor initial real-time intra operative, TRUS-derived prostate volume are prognostic for post-procedural volume change. The magnitude of real-time intraoperative prostate volume change between initiation and completion of the implant may be associated with the day 30 prostate volume change.