The accuracy of interpolation or averaging procedures for obtaining the calibration coefficient NK for 192Ir high-dose-rate brachytherapy sources has been investigated using the EGSnrc Monte Carlo simulation system. It is shown that the widely used two-point averaging procedure of Goetsch et al. [Med. Phys. 18, 462 (1991)] has some conceptual problems. Most importantly, they recommended, as did the IAEA, averaging A wallNK values whereas one should average 1/NK values. In practice this and other issues are shown to have little effect except for Goetsch et al.'s methods for determining Awall values. Their method of generalizing the Awall values measured in one geometry to other geometries is incorrect by up to 2%. However, these errors in A wall values cause systematic errors of only 0.3% in 192Ir calibration coefficients. It is shown that Awall values need not be included in the averaging technique at all, thereby simplifying the technique considerably. It is demonstrated that as long as ion chambers with a flat response are used and/or very heavily filtered 250 kV (or higher) beams of x rays are used in the averaging, then almost all techniques can provide adequate accuracy.

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Medical Physics
Department of Physics

Mainegra-Hing, E. (Ernesto), & Rogers, D.W.O. (2006). On the accuracy of techniques for obtaining the calibration coefficient NK of 192Ir HDR brachytherapy sources. Medical Physics, 33(9), 3340–3347. doi:10.1118/1.2239198