DOSIMETRY OF YB-169 SEED MODEL X1267

Unlike previous brachytherapy sources a number of published studies ha ve been addressed to the dosimetry of Yb-169 seeds, manufactured in se veral prototypes, before widespread clinical use has been made. Discre pancies seen in the dosimetry obtained for ytterbium seed prototypes a ppear to be related to inconsistency and non-reproducibility in the ve ndor's calibration procedure to determine contained activity, A(v). Th e comparison of Yb-169 seed dosimetries demonstrates a need for more a ccurate implementation of calibration procedures to determine the air kerma rate for the definitive Yb-169 seed design. This paper reports a n experimental procedure to determine the reference air kerma rate, K- r (mu Gy h(-1)), defined as the kerma rate at 1 m along the source tra nsverse axis in free space for the new Yb-169 seed, model X1267. A mea n value of the ratio K-r/A(v) = 1.53 mu Gy h(-1) mCi(-1) was obtained from determining the K-r value of eleven seeds. Since this ratio is on ly 3% less than the air kerma rate constant for the Yb-169 point sourc e, (Gamma delta)(k) = 1.58 mu Gy h(-1) m(2) mCi(-1), this means that t he A(v) is closer to an apparent activity than a contained activity, A (c). A Monte Carlo simulation to determine the ratio between reference air kerma rate and the contained activity gave K-r/A(c) = 1.33 mu Gy h(-1) mCi(-1). For the dose rate constant in water we obtained D-Kr(1, pi/2) = 1.20 +/- 0.05 cGy h(-1) (mu Gy h(-1))(-1), using calibrated t hermoluminescent dosimeters (TLDs) and D-Kr(1, pi/2) = 1.21 +/- 0.03 c Gy h(-1) (mu Gy h(-1))(-1) by Monte Carlo simulation. TLDs were used b oth to determine the radial dose distribution along the seed transvers e axis and to calibrate GAFChromic films to obtain the two-dimensional dose distribution around the seed.