MR and CT image fusion for postimplant analysis in permanent prostate seed implants

Purpose: To compare the outcome of two different image-based postimplant dosimetry methods in permanent seed implantation. Methods and Materials: Between October 1999 and October 2002, 150 patients with low-risk prostate carcinoma were treated with I-125 and Pd-103 in our institution. A CT-MRI image fusion protocol was used in 21 consecutive patients treated with exclusive brachytherapy. The accuracy and reproducibility of the method was calculated, and then the CT-based dosimetry was compared with the CT-MRI-based dosimetry using the dose-volume histogram (DVH) related parameters recommended by the American Brachytherapy Society and the American Association of Physicists in Medicine. Results: Our method for CT-MRI image fusion was accurate and reproducible (median shift <1 mm). Differences in prostate volume were found, depending on the image modality used. Quality assurance DVH-related parameters strongly depended on the image modality (CT vs. CT-MRI): V-100 = 82% vs. 88%, p < 0.05. D-90 = 96% vs. 115%, p < 0.05. Those results depend on the institutional implant technique and reflect the importance of lowering inter- and intraobserver discrepancies when outlining prostate and organs at risk for postimplant dosimetry. Conclusions: Computed tomography-MRI fused images allow accurate determination of prostate size, significantly improving the dosimetric evaluation based on DVH analysis. This provides a consistent method to judge a prostate seed implant's quality. (C) 2004 Elsevier Inc.