Dose rate constant and energy spectrum of interstitial brachytherapy sources

In the past two years, several new manufacturers have begun to market low-energy interstitial brachytherapy seeds containing I-125 and Pd-103. Parallel to this development, the National Institute of Standards and Technology (NIST) has implemented a modification to the air-kerma strength (S-K) standard for I-125 seeds and has also established an S-K standard for Pd-103 seeds. These events have generated a considerable number of investigations on the determination of the dose rate constants (Lambda) of interstitial brachytherapy seeds. The aim of this work is to study the general properties underlying the determination of Lambda and to develop a simple method for a quick and accurate estimation of Lambda. As the dose rate constant of clinical seeds is defined at a fixed reference point, we postulated that Lambda may be calculated by treating the seed as an effective point source when the seed's source strength is specified in S-K and its source characteristics are specified by the photon energy spectrum measured in air at the reference point. Using a semi-analytic approach, an analytic expression for Lambda was derived for point sources with known photon energy spectra. This approach enabled a systematic study of Lambda as a function of energy. Using the measured energy spectra, the calculated Lambda for I-125 model 6711 and 6702 seeds and for Ir-192 Seed agreed with the AAPM recommended values within +/-1%. For the Pd-103 model 200 seed, the agreement was 5% with a recently measured value (within the +/-7% experimental uncertainty) and was within 1% with the Monte Carlo simulations. The analytic expression for Lambda proposed here can be evaluated using a programmable calculator or a simple spreadsheet and it provides an efficient method for checking the measured dose rate constant for any interstitial brachytherapy seed once the energy spectrum of the seed is known. (C) 2001 American Association of Physicists in Medicine.