MO‐D‐AUD B‐02: Dose Rate Constants Determined by a Photon Spectrometry Technique for 20 Different Models of Low‐Energy Brachytherapy Sources

Purpose: To perform a systematic and independent determination of the dose rate constants (Λ) of available low‐energy interstitial brachytherapy sources using a recently developed photon spectrometry technique (PST). Method and Materials: A total of 60 low‐energy interstitial brachytherapy sources (20 different models with 3 sources per model) containing either125 I (14 models),103 Pd (5 models), or131 Cs (one model) were included in this study. A recently developed photon spectrometry technique ( Med. Phys. 34 , 1412–1430, 2007) was used to determine the PST Λ for each source. Source‐dependent variations in PST Λ were analyzed systematically against the spectral characteristics of the emitted photons and the AAPM consensus values ( CON Λ) when available. Results: The PST Λ determined for the103 Pd ,125 I , and131 Cs sources had values of 0.661 to 0.678, 0.959 to 1.024, and 1.066 cGyh −1 U −1 , respectively. The variation in PST Λ among the 5103 Pd source models was less than 3%; due mainly to the variations in spatial distribution of radioactivity. The variation in PST Λ among the 14125 I source models was larger and the maximum difference was over 6%. These variations were caused primarily by the presence of silver in some source models and, to a lesser degree, by the variations in activity distribution. When silver was present, the PST Λ exhibited strong dependence on the silver content with values varying from 0.959 to 1.019 cGyh −1 U −1 . When silver was absent, the PST Λ was less variable and had values within 1% of 1.024 cGyh −1 U −1 . The PST Λ was found within 2% (14 models) and 2.6% (one model) of CON Λ for 15 models current have such a value. Conclusion: Excellent agreement between PST Λ and CON Λ was observed for all source models that currently have an AAPM consensus value. These results demonstrate that the PST is an accurate and robust technique for the determination of Λ for low‐energy brachytherapy sources.