Dose distributions and dose rate constants for new ytterbium-169 brachytherapy seeds

Relative dose distributions in the vicinities of two new prototypes of ytterbium-169 brachytherapy sources have been measured using lithium fluoride thermoluminescent dosimeters (TLD) placed in a solid water phantom. The type 6 seed consists of four Yb2O3 spheres contained in a 0.075-mm thick titanium tube, with overall dimensions of 4.5 mm in length and 0.8 mm in diameter. The type 8 seed contains a cylindrical segment of Yb wire, 2.5 mm long, sealed in a 0.075-mm thick titanium tube 4 mm in length and 0.5 mm in diameter. The relative dose distributions, measured with TLD, were compared with those predicted by Monte Carlo simulations (MCNP code). Experimental and theoretical dose distributions were generally in agreement within 5% of the local dose value. Source strength was determined from activity measurements using a high purity germanium (HPGe) spectrometer. With source strengths established, thermoluminescence was then used to measure the dose rate constant, lambda 0, for the type 8 seed. The measured lambda 0 of 1.34 +/- 0.10 cGy U-1 for the type 8 seed agrees, within statistical uncertainty, with the value of 1.25 +/- 0.05 cGy U-1 predicted through Monte Carlo simulation. Comparisons are made with experimental data reported by other investigators.