Postimplant rectal dosimetry is not dependent on 103Pd or 125I seed activity

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.

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 for (125)I and ≤1.330, 1.331-1.547, and >1.547 mCi/seed for (103)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 (R(100)) was calculated in cubic centimeters. The prostate V(100) and D(90) volumes were also calculated.

The mean prostate volume was 35.8 and 32.3 cm(3) for (125)I and (103)Pd. The median time to postimplant CT was 30 days. For (125)I, 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 (125)I 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 (103)Pd vs. (125)I (0.63 vs. 0.82 cm(3), p<0.001).

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 (125)I, whereas (103)Pd dosimetry was not dependent on seed activity.