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

The National Institute of Standards and Technology (NIST) introduced a national standard for air kerma strength of the ThreaSeed Model 200 103Pd source (the only 103Pd seed available until 1999) in early 1999. Correct implementation of the NIST-99 standard requires the use of dose rate constants normalized to this same standard. Prior to the availability of this standard, the vendor's calibration procedure consisted of intercomparing Model 200 seeds with a 109Cd source with a NIST-traceable activity calibration. The AAPM undertook a comprehensive review of 103Pd source dosimetry including (i) comparison of the vendor and NIST-99 calibration standards; (ii) comparison of original Task Group 43 dosimetry parameters with more recent studies; (iii) evaluation of the vendor's calibration history; and (iv) evaluation of administered-to-prescribed dose ratios from the introduction of 103Pd sources in 1987 to the present. This review indicates that for a prescribed 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 correctly the 1999 dosimetry data and NIST-99 standard for 103Pd source: (1) the vendor should provide calibrations in terms of air kerma strength traceable to NIST-99 standard, (2) the medical physicist should update the treatment planning system with properly normalized (to NIST-99) dosimetry parameters for the selected 103Pd source model, and (3) the radiation oncologist in collaboration with the medical physicist should decide which clinical experience they wish to duplicate; the one prior to 1997 or the one from 1997 to 1999. If the intent is to duplicate the experience prior to 1997, which is backed by the long-term follow-up and published outcome studies, then the prior prescriptions of 115 Gy should be replaced by 124 Gy to duplicate that experience.