Postimplant rectal dosimetry is not dependent on103 Pd or125 I seed activity

Abstract 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 [125 I] and 737 palladium-103 [103 Pd]) 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 for125 I and ≤1.330, 1.331–1.547, and >1.547 mCi/seed for103 Pd). 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 ( R100 ) was calculated in cubic centimeters. The prostate V100 and D90 volumes were also calculated. Results The mean prostate volume was 35.8 and 32.3 cm3 for125 I and103 Pd. The median time to postimplant CT was 30 days. For125 I, the V100 increased from 91.0% to 93.7% ( p = 0.012) and the D90 increased from 105.9% to 108.7% ( p < 0.001) for the lowest to the highest125 I seed activities. In contrast, no significant changes in V100 ( p = 0.751) or D90 ( p = 0.200) were discerned when stratified by seed activity. For both isotopes, there was no correlation between seed activity and R100 , and R100 was highest for the intermediate seed activities. Overall, the R100 was lower for103 Pd vs.125 I (0.63 vs. 0.82 cm3 , 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 for125 I, whereas103 Pd dosimetry was not dependent on seed activity.