A secondary air kerma strength standard for Yb-169 interstitial brachytherapy sources

Ytterbium-169 (¹⁶⁹Yb) is promising new intermediate low-energy isotope 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. The authors have solved this problem by adapting the free air measurement technique of Goetsch et al., originally developed for ¹⁹²Ir (see Med. Phys., vol. 18, p.462-7, 1991). Using a 100 cm³ spherical ion chamber with NIST traceable external beam calibrations in a free air geometry, the authors have measured the air kerma strength of 6 different source batches (2 type 6 batches, 3 type 8 batches, and 1 experimental high-intensity source). Room scatter corrections, 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 cm² mCi^-1 h^-1 (0.0420 mu Gy m² MBq^-1 h^-1), in conflict with the expected 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 as 10% whereas the S_K/dose calibrator reading ratio varies by no more than 0.3%, suggesting that the reproducibility of A_vendor is relatively poor. These discrepancies may help explain the variation (as large as 28%) in published dose rate constants for ¹⁶⁹Yb.