title: On the accuracy of techniques for obtaining the calibration coefficient N(K) of 192Ir HDR brachytherapy sources.
abstract: The accuracy of interpolation or averaging procedures for obtaining the calibration coefficient N(K) 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(wall)N(K) values whereas one should average 1/N(K) values. In practice this and other issues are shown to have little effect except for Goetsch et al.'s methods for determining A(wall) values. Their method of generalizing the A(wall) 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 A(wall) 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.
authors.affiliation: Ionizing Radiation Standards, National Research Council of Canada, Ottawa K1A 0R6, Canada.
authors.forename: Ernesto
authors.initials: E
authors.lastname: Mainegra-Hing
authors.forename: D W O
authors.initials: DW
authors.lastname: Rogers
mesh_terms: D001918:Brachytherapy / Q000295:instrumentation* / Q000379:methods*; D002138:Calibration; D004867:Equipment Design; D019544:Equipment Failure Analysis; D007496:Iridium Radioisotopes / Q000032:analysis* / Q000592:standards*; D011874:Radiometry / Q000379:methods* / Q000592:standards*; D011879:Radiotherapy Dosage; D012016:Reference Values; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
chemical_list: D007496:Iridium Radioisotopes