Impact of transrectal ultrasound- and computed tomography-based seed localization on postimplant dosimetry in prostate brachytherapy

PURPOSE: To study the impact of seed localization, as performed by different observers using linked I-125 seeds, on postimplant dosimetry in prostate brachytherapy and, to compare transrectal ultrasound (TRUS)-based with CT-based approach for the dosimetric outcomes.METHODS AND MATERIALS: Nineteen permanent prostate implants were conducted using linked I-125 seeds. Postimplant TRUS and CT images were acquired and prostate glands were, after implantation, delineated on all images by a single oncologist, who had performed all 19 seeding procedures. Six observers independently localized the seeds on both TRUS and CT images, from which the principle dosimetric parameters V-100 (volume of prostate that received the prescribed dose), V-150 (volume of prostate that received 150% of the prescribed dose), and D-90 (minimal dose delivered to 90% of the prostate) were directly calculated for each patient. A single-factor analysis of variance was first applied to determine interobserver variability in seed localization. A nonparametric comparison of the approach using TRUS and CT was then carried out by the Wilcoxon paired-sample test.RESULTS: Analysis from the analysis of variance for TRUS showed that the null hypothesis for equal means, could not be rejected for all six observers based on a significance level alpha = 0.05. TRUS-based and CT-based approaches were then cross compared by the Wilcoxon paired-sample test, which suggested that the null hypothesis was insignificant for V-100 and D-90, but was significant for V-150.CONCLUSIONS: Both TRUS- and CT-imaging modalities provided indistinguishable postimplant dosimetry results as far as V-100 and D-90 were concerned. There was comparable observer independence between TRUS- and CT-based seed localization for linked-seed implant procedures. With other advantages that TRUS-imaging modality had over CT in the evaluation of postimplant dosimetry, TRUS would be a preferred choice in conjunction with linked seeds for intraoperative procedures in prostate brachytherapy. (C) 2009 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved.