Thermoluminescent dosimetry of the SourceTech Medical model STM1251 ¹²⁵I seed

Many new models of ¹²⁵I seeds are being introduced, mainly due to the increase in prostate seed implants. We have evaluated the SourceTech Medical (STM), model STM1251, ¹²⁵I seed using thermoluminescent dosimeters (TLDs) in a solid water phantom. TLD cubes, LiF TLD-100, with dimension 1 mm on each edge, were irradiated at various distances, 1, 2, 3, and 5 cm, at angles ranging from 0° to 90° in 10° increments. Sensitivity calibration of the TLDs was achieved by irradiation to 10 cGy with 6 MV x rays from a clinical linear accelerator, Clinac 600C. Concurrent with the ¹²⁵I seed exposures, several TLDs were also exposed to 10 cGy with the 600C as a control set. Dose rates per unit air kerma strength were determined based on the 1999 NIST traceable standard for the STM1251 seed. They are presented as a function of distance r and angle θ. The TG-43 parameters, including the dose rate constant, Λ, anisotropy function, F(r, θ), radial dose function, g(r), anisotropy factor, φ_an(r), and anisotropy constant, φ, were obtained for use in radiation treatment planning software. The value of A was determined as 1.07 ± 5.5% cGy U^-1 h^-1, which is comparable to model 6702 and to the value determined using the point extrapolation method by Kirov and Williamson. We also find agreement between our TLD data and their Monte Carlo results for g(r), F(r, φ), φ_an(r), and φ. Additionally, agreement is found with the TLD data of Li and Williamson for Λ and g(r).