Dynamic intraoperative dosimetry for prostate brachytherapy using a nonisocentric C-arm

PURPOSE: To evaluate a prototypical system of dynamic intraoperative dosimetry for prostate brachytherapy using registered ultrasound and fluoroscopy (RUF) with a nonisocentric C-arm (GE OEC, Salt Lake City, UT) and to compare intraoperative dosimetry of RUF as well as ultrasound-based seed localization (USD) with Day 0 CT dosimetry. METHODS: Seed positions were independently determined using RUF and USD. RUF uses a radio-opaque fiducial for registration to ultrasound and 3-dimensional reconstruction of seeds relative to prostate using nonisocentric C-arm fluoroscopy. Postimplant CT was performed on Day 0. Squared differences between dosimetric measures for RUF vs. CT and USD vs. CT were calculated and mean squared differences evaluated. Paired t test was used to evaluate which method was more closely aligned with CT. Accuracies of USD and RUF compared with CT were estimated using a nonparametric approach. RESULTS: Six patients were treated and compared with USD. RUF identified areas of underdosage intraoperatively in all patients and median 5 additional seeds were placed. In 40 of 42 measures, RUF was equally or more closely correlated with CT than USD. USD showed statistically significant variation from CT for 6 of 7 parameters compared with I of 7 parameters for RUF. Mean squared differences from CT were significantly smaller for RUF in 4 of 7 parameters compared with USD. CONCLUSIONS: Dynamic intraoperative dosimetry is possible with a conventional nonisocentric C-arm. Compared with an USD method, RUF-based intraoperative dosimetry was more closely aligned with immediate postimplant CT. RUF identified areas of underdosage, which were not detected using USD. (C) 2011 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved.