Toward a statistically relevant calibration end point for prostate seed implants

Interstitial brachytherapy for carcinoma of the prostate is achieved throug h the use of a configuration of radioactive seeds placed in a manner that d elivers a customized, reasonably uniform dose to the target volume. Accurat e dose delivery depends on both precise seed placement and reliable seed st rength in the implanted configuration. This study assumes the independence of the two issues, and quantifies the reduction in the minimum dose to the surface of the grand due only to variability in individual seed strengths. Current AAPM guidelines pertaining to the acceptable limits on seed-to-seed variability are prudent for small configurations of seeds, yet are likely to be overly stringent for applications such as prostate seed implantation. In this study we determine the reduction in the minimum peripheral dose (m PD) caused by the introduction of source strength variability, and provide statistical insight into this effect. It is concluded that the current guid elines limit the reduction in mPD to less than or equal to 0.44% relative t o the prescription value, for an average configuration, due to the inclusio n of strength variability. The maximum observed reduction in mPD would be l ess than or equal to 1.5%. This value is an order of magnitude lower than t he recommendations of the AAPM Task Group 40 for the overall accuracy of br achytherapy procedures, which suggests that seed strength variability is of limited concern and that constraints on this factor should perhaps be reev aluated. (C) 2000 American Institute of Physics.[S0093-2405(00)00701-X].