Dosimetry of 169 Yb seed model X1267.

Unlike previous brachytherapy sources a number of published studies have been addressed to the dosimetry of 169 Yb seeds, manufactured in several prototypes, before widespread clinical use has been made. Discrepancies seen in the dosimetry obtained for ytterbium seed prototypes appear to be related to inconsistency and non-reproducibility in the vendor's calibration procedure to determine contained activity. Av. The comparison of 169 Yb seed dosimetries demonstrates a need for more accurate implementation of calibration procedures to determine the air kerma rate for the definitive 169 Yb seed design. This paper reports an experimental procedure to determine the reference air kerma rate, Kr (mu Gy h-1), defined as the kerma rate at 1 m along the source transverse axis in free space for the new 169 Yb seed, model X1267. A mean value of the ratio Kr/Av = 1.53 mu Gy h-1 mCi-1 was obtained from determining the Kr value of eleven seeds. Since this ratio is only 3% less than the air kerma rate constant for the 169 Yb point source, (gamma delta)k = 1.58 mu Gy h-1 m2 mCi-1, this means that the Av is closer to an apparent activity than a contained activity, Ac. A Monte Carlo simulation to determine the ratio between reference air kerma rate and the contained activity gave Kr/Ac = 1.33 mu Gy h-1 mCi-1. For the dose rate constant in water we obtained DKr (1, pi/2) = 1.20 +/- 0.05 cGy h-1 (mu Gy h-1)-1, using calibrated thermoluminescent dosimeters (TLDs) and DKr (1, pi/2) = 1.21 +/- 0.03 cGy h-1 (mu Gy h-1)-1 by Monte Carlo simulation. TLDs were used both to determine the radial dose distribution along the seed transverse axis and to calibrate GAFChromic films to obtain the two-dimensional dose distribution around the seed.