Air-kerma strength determination of a 169Yb high dose rate brachytherapy source

The increased demand for high dose rate (HDR) brachytherapy as an alternative to external beam radiotherapy has led to the introduction of a HDR brachytherapy isotope Yb-169. This source offers a dose rate similar to Ir-192 HDR sources, at about one fourth the effective photon energy. This work presents the calibration of this source in terms of air-kerma strength, based on an adaptation of the current, National Institute of Standards and Technology traceable, in air measurement technique currently used for Ir-192 HDR sources. Several additional measurement correction factors were required, including corrections for air scatter, air attenuation, and ion recombination. A new method is introduced for determining the ion chamber calibration coefficient (NkYb)-Yb-169. An uncertainty analysis was also performed, indicating an overall measurement expanded uncertainty in the air-kerma strength (k=2) of 2.2%. (C) 2008 American Association of Physicists in Medicine.