Recommendations of the American Association of Physicists in Medicine on 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 source (the only 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 source with a NIST‐traceable activity calibration. The AAPM undertook a comprehensive review of 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 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