Recommendations of the American Association of Physicists in Medicine on Pd-103 interstitial source calibration and dosimetry: Implications for dose specification and prescription

The National Institute of Standards and Technology (NIST) introduced a nati onal standard for air kerma strength of the ThreaSeed(R) Model 200 Pd-103 s ource (the only Pd-103 Seed available until 1999) in early 1999. Correct im plementation of the NIST-99 standard requires the use of dose rate constant s normalized to this same standard. Prior to the availability of this stand ard, the vendor's calibration procedure consisted of intercomparing Model 2 00 seeds with a Cd-109 source with a NIST-traceable activity calibration. T he AAPM undertook a comprehensive review of Pd-103 source dosimetry includi ng (i) comparison of the vendor and NIST-99 calibration standards; (ii) com parison of original Task Group 43 dosimetry parameters with more recent stu dies; (iii) evaluation of the vendor's calibration history; and (iv) evalua tion of administered-to-prescribed dose ratios from the introduction of Pd- 103 sources in 1987 to the present. This review indicates that for a prescr ibed dose of 115 Gy, the administered doses were (a) 124 Gy for the period 1988-1997 and (b) 135 Gy for the period 1997-1999. The AAPM recommends that the following three steps should be undertaken concurrently to implement c orrectly the 1999 dosimetry data and NIST-99 standard for Pd-103 source: (1 ) the vendor should provide calibrations in terms of air kerma strength tra ceable to NIST-99 standard, (2) the medical physicist should update the tre atment planning system with properly normalized (to NIST-99) dosimetry para meters for the selected Pd-103 source model, and (3) the radiation oncologi st in collaboration with the medical physicist should decide which clinical experience they wish to duplicate; the one prior to 1997 or the one from 1 997 to 1999. If the intent is to duplicate the experience prior to 1997, wh ich is backed by the long-term follow-up and published outcome studies, the n the prior prescriptions of 115 Gy should be replaced by 124 Gy to duplica te that experience. (C) 2000 American Association of Physicists in Medicine .