Dosimetric characteristics of the type 8 ytterbium-169 interstitial brachytherapy source

The type 8 ytterbium-169 seed is an intermediate low-energy (93 keV) brachytherapy source recently introduced as a temporary interstitial source. Thermoluminescent dosimetry (TLD) was used to confirm the absolute dose-rate predictions of a Monte Carlo photon transport (MCPT) code. Limited LiF TLD- 100 readings in a solid-water phantom were obtained to characterize the transverse axis (0.5 to 7 cm) and two-dimensional polar dose-rate distributions. Precision diode measurements in a liquid-water phantom confirmed MCPT relative dose estimates at distances as small as 2 mm from the source center. An MCPT code accurately modeled the source, phantom, and detector geometry simulated the response of each irradiated dosimeter. The experimental and theoretical results were found to agree within the estimated experimental uncertainty, confirming that Monte Carlo simulation is an accurate tool for characterizing dose-rate distributions of brachytherapy sources in this energy range. Using the validated MCPT code, the two- dimensional dose-rate distribution of the type 8 seed was calculated for a source centered in a 30-cm liquid-water sphere. An away-and-along lookup table is provided as well as the dose-rate constant, radial dose function, and anisotropy function.