A method for implementing the American Association of Physicists in Medicine Task Group-43 dosimetry recommendations for 125I transperineal prostate seed implants on commercial treatment planning systems.

Purpose: To present our experience implementing the American Association of Physicists in Medicine (AAPM) Task Group #43 (TG-43) recommendations for 125I dosimetry parameters utilizing two commercially available treatment planning systems.

Methods And Materials: Single seed point source calculations were performed on two treatment planning systems and intercomparisons were conducted between known points of interest for (a) previously published data on each system, (b) TG-43 data on each system, and (c) hand calculations. Additional calculations and intercomparisons were performed for a typical 35 cm3 prostate volume implant with a prescribed dose of 160 Gy.

Results: Intercomparisons of calculations for single seed point source approximations with previously published data and TG-43 data indicated good agreement between the two systems at a distance of 1 cm. Point source comparisons between previously published data and TG-43 recommendations show a difference in dose of 11% to the 1 cm point. Calculations for a 35 cm3 permanent interstitial prostate implant showed the 160 Gy isodose line pulled centrally toward the seeds 1-2 mm as a result of the 11% change.

Conclusions: It is advisable for physicians, physicists, and dosimetrists to exercise caution when incorporating the TG-43 recommendations for 125I into treatment planning systems. The effects caused by differences in dosimetry parameters need to be understood and accounted for when matching prescribed doses in order to ensure continuity of treatment and clinical results as reported in the current literature.