A SECONDARY AIR KERMA STRENGTH STANDARD FOR YB-169 INTERSTITIAL BRACHYTHERAPY SOURCES

Ytterbium-169 (Yb-169) is a promising new intermediate low-energy isot ope for interstitial implantation. To date, no air kerma strength (S-K ) standard for this source exists that can serve as a sound foundation for comparing various dose measurements and theoretical calculations. We have solved this problem by adapting the free air measurement tech nique of Goetsch et al, originally developed for Ir-192. Using a 100 c m(3) spherical ion chamber with NIST traceable external beam calibrati ons in a free air geometry, we have measured the air kerma strength of six different source batches (two type 6 batches, three type 8 batche s, and one experimental high-intensity source). Room scatter correctio ns, derived from an empirical fit to the data (following Goetsch et al ) and/or directly by Monte Carlo simulation, yielded identical results with a reproducibility of 1%. The ratio [S-K/A(vendor)] Of measured S -K to the vendor's contained activity assay averaged 1.554 cGy cm(2) m Ci(-1) h(-1) (0.0420 mu Gy m(2) MBq(-1) h(-1)), in conflict with the e xpected value of 1.34 (0.0362), derived from Monte Carlo calculations. The measured [S-K/A(vendor)] for the type 8 seeds varies by as much a s 10% whereas the S-K/dose calibrator reading ratio varies by no more than 0.3%, suggesting that the reproducibility of A(vendor) is relativ ely poor. These discrepancies may help explain the variation (as large as 28%) in published dose rate constants for Yb-169.