Photon energy spectrum emitted by a novel polymer-encapsulated 103Pd source and its effect on the dose rate constant

Two independent groups have published intrinsic dosimetry parameters for the recently introduced OptiSeed103 interstitial brachytherapy source which contains 103Pd encapsulated by a novel polymer shell. The dose rate constant (Lambda) reported by the two groups, however, differed by more than 6% and there is currently no AAPM recommended consensus value for this source in clinical dosimetry. The aim of this work was to perform an independent determination of Lambda for the OptiSeed103 source using a recently developed photon spectrometry technique. Three OptiSeed103 sources (model 1032P) with known air-kerma strength were used in this study. The photon energy spectrum emitted along the radial direction on the source's bisector was measured in air using a high-resolution intrinsic germanium spectrometer designed and established for low-energy brachytherapy source spectrometry. The dose rate constant of each source was determined from its emitted energy spectrum and the spatial distribution of radioactivity in the source. Unlike other sources made with traditional titanium encapsulation, the photons emitted by the OptiSeed103 sources exhibited only slight spectral hardening, yielding a relative energy spectrum closer to that emitted by bare 103Pd. The dose rate constant determined by the photon spectrometry technique for water was 0.664 +/- 0.025 cGy h(-1) U(-1). This value agreed, within experimental uncertainties, with the Monte Carlo-calculated value (MCLambda) of 0.665 +/- 0.014 cGy h(-1) U(-1) and the TLD-measured value (with a Monte Carlo-calculated solid-phantom-to-water conversion factor) of 0.675 +/- 0.051 cGy h(-1) U(-1) reported by Wang and Hertel [Appl. Radiat. Isot. 63, 311-321 (2005)]. However, it differed by -6.7% from the McLambda of 0.712 +/- 0.043 cGy h(-1) U(-1) reported by Bernard and Vynckier [Phys. Med. Biol. 50, 1493-1504 (2005)]. The results obtained in this work provide additional information needed for establishing a consensus value for the dose rate constant for the OptiSeed103 source. It suggests that an eventual consensus value of Lambda for the OptiSeed103 source is likely to be closer to a value of 0.668 cGy h(-1) U(-1) rather than 0.693 cGy h(-1) U(-1) as initially recommended by the source manufacturer based on the two previously published results.