AbstractID: 8068 Title: General Methods for Calculating the Air Kerma Rate for Clinical Dosimetry of Low-Energy
Photon Emitting Brachytherapy Sources
The AAPM recommends use of air kerma strength (SK) instead of apparent activity for clinical applications. SK is
used with the dose rate constant (Λ) and other dosimetry parameters to calculate brachytherapy patient dose. While
SK may be determined through either experimental measurements or radiation physics calculations, there are no
recommended methodologies for how investigators should evaluate SK or Λ. The aim of this work was to present a
few different calculation methodologies to evaluate SK for a generalized brachytherapy source, and propose an
optimal methodology to best adhere to the definition of SK for calculating Λ. These methodologies are summarized:
(1) shutoff Ti x-ray production and integrate results over all photon energies, (2) retain Ti x-ray production, but
remove their relative dosimetric enhancements offline, (3) create a high-pass energy shell around the brachytherapy
source that prevents passage of photons with E < 5 keV, and (4) calculate photon energy fluence in free space so that
effects of attenuation and scatter by air never arise, but remove source photons with E < 5 keV. Method (1) is that
historically used by Williamson, while Rivard recently introduced method (2). As expected, differences were
relatively small (~1%) between results obtained using these four methodologies. However, larger differences are
expected for non-generalized sources such as those routinely used in clinical brachytherapy. Since a recommended
methodology for determination of SK and Λ is desirable, we suggest use of method (4) because it best adheres to the
definition of SK.