Measurement of dose‐rate constant for seeds with air kerma strength calibration based upon a primary national standard

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Abstract
Recent developments in the past two years require a significant change in the dosimetry of brachytherapy sources (Theraseed® model 200, manufactured by Theragenics Corp., Atlanta, GA). Since their introduction in 1987, the air kerma strength of sources for interstitial brachytherapy has been determined using a system of apparent activity measurement based upon the measurement of photon fluence at a reference distance along the transverse axis of the source free in air, using a NaI (T1) scintillation detector at the manufacturer's facilities. This detection system has been calibrated against a National Institute of Standards and Technology (NIST)‐traceable activity standard of a source. This system produced a highly consistent standard (within ±2%) for over 12 years, with the exception of the last source change in September 1997, which resulted in a change of 9% from the original 1987 standard. The second major development affecting dosimetry is that on 13 January 1999 a primary national standard for the air kerma strength of seeds was developed by NIST. This primary standard is based upon an absolute measurement of air kerma rate free in air at a reference distance from the source along its transverse axis using a wide angle free air chamber (WAFAC). In order to implement this new standard for the calibration of source strength in clinical dosimetry for interstitial implants, it is necessary to measure the dose‐rate constant for the seeds using a calibration of source strength based on the NIST 99 standard. In this work, a measurement of the dose‐rate constant using lithium fluoride (LiF) thermoluminescent dosimeters (TLDs) in a water equivalent solid phantom is reported. The measured value of this constant is where the unit air kerma strength is and is directly traceable to the NIST 99 standard. The implementation of the NIST 99 standard for should be accompanied by a simultaneous adoption of the new dose‐rate constant reported here. No changes in radial dose function, anisotropy function, anisotropy factor, and geometry function are needed. However, a change in prescribed dose may be necessary to deliver the same physical dose as before.