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 125I and 103Pd. Parallel to this development, the National Institute of Standards and Technology (NIST) has implemented a modification to the airkerma strength (SK) standard for 125I seeds and has also established an SK standard for 103Pd 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 seedʹs source strength is specified in SK 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 lambdaas a function of energy.