Postimplant Rectal Dosimetry Is Not Dependent On Pd-103 Or I-125 Seed Activity

PURPOSE: In this study, the effect of prostate brachytherapy seed activity on postimplant rectal dosimetry was evaluated in Pro-Qura (Prostate Brachytherapy Quality Assurance; Seattle, WA) proctored, community-based programs.METHODS AND MATERIALS: Twenty-three hundred patients (1563 iodine-125 [I-125] and 737 palladium-103 [Pd-103]) from 78 brachytherapists with postimplant rectal dosimetry were identified. Seed activity was stratified into three tertiles for each isotope (<= 0.300, 0.301-0.326, and >0.326 mCi/seed for I-125 and <= 1.330, 1.331-1.547, and >1.547 mCi/seed for Pd-103). Postimplant dosimetry was performed in a standardized fashion. The rectum was contoured by outlining the outer rectal wall. The volume of the rectum receiving 100% of the prescription dose (R-100) was calculated in cubic centimeters. The prostate V-100 and D-90 volumes were also calculated.RESULTS: The mean prostate volume was 35.8 and 32.3 cm(3) for I-125 and Pd-103. The median time to postimplant CT was 30 days. For I-125, the V-100 increased from 91.0% to 93.7% (p = 0.012) and the D-90 increased from 105.9% to 108.7% (p < 0.001) for the lowest to the highest I-125 seed activities. In contrast, no significant changes in V-100 (p = 0.751) or D-90 (p = 0.200) were discerned when stratified by seed activity. For both isotopes, there was no correlation between seed activity and R-100, and R-100 was highest for the intermediate seed activities. Overall, the R-100 was lower for Pd-103 vs. I-125 (0.63 vs. 0.82 cm(3), p < 0.001).CONCLUSIONS: Within the confines of seed activities used in this study, higher activity seeds did not result in a deleterious effect on rectal dose. Higher activity seeds were associated with improved prostate dosimetry for I-125, whereas Pd-103 dosimetry was not dependent on seed activity. (C) 2011 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved.