Thermoluminescent dosimetry of the Symmetra™ ¹²⁵I model I25.S06 interstitial brachytherapy seed

As the efficacy of brachytherapy prostate treatment is becoming realized, new models of ¹²⁵I seeds are being introduced. In this article we present thermoluminescent dosimetry (TLD) in a solid water phantom for a new design of ¹²⁵I seed (UroMed/Bebig Symmetra™, Model I25.S06). TLD cubes, LiF TLD-100, from Bicron (Solon, OH) with dimension 1×1×1 mm³ were irradiated at various distances from the seed at angles ranging from 0° to 90° in 10° increments. The TLD detectors were calibrated by irradiation in a ⁶⁰Co teletherapy beam. Monte Carlo simulation was used to account for TLD energy dependence and the deviation of solid water composition (as determined by chemical analysis of a sample) from liquid water, Dose rates per unit air kerma strength were determined based on calibrations traceable to the 1999 NIST standard (corrected for NIST measurement errors made in 1999) for the Symmetra™ seed. Dose data is presented in TG-43 format as a function of distance and angle, Values for Λ, F(r, θ), g(r), and the anisotropy constant are obtained for use in radiation treatment planning (RTP) software. The dose rate constant was determined to be 1.033 ± 6.4% cGy h^-1 U^-1, which is comparable to model 6702 and higher than model 6711. We find the relative dose distributions of the Symmetra™ seed are similar to model 6702, and less anisotropic than model 6711, After accounting for deviation of measured solid water composition from the manufacturer's specification, good agreement between TLD results and Monte-Carlo-aided values was found.