A radiation quality correction factor <i>k_Q</i> for well‐type ionization chambers for the measurement of the reference air kerma rate of ⁶⁰Co HDR brachytherapy sources

<jats:sec><jats:title>Purpose:</jats:title><jats:p>The aim of this study was to investigate whether a chamber‐type‐specific radiation quality correction factor <jats:italic>k<jats:sub>Q</jats:sub></jats:italic> can be determined in order to measure the reference air kerma rate of <jats:sup>60</jats:sup>Co high‐dose‐rate (HDR) brachytherapy sources with acceptable uncertainty by means of a well‐type ionization chamber calibrated for <jats:sup>192</jats:sup>Ir HDR sources.</jats:p></jats:sec><jats:sec><jats:title>Methods:</jats:title><jats:p>The calibration coefficients of 35 well‐type ionization chambers of two different chamber types for radiation fields of <jats:sup>60</jats:sup>Co and <jats:sup>192</jats:sup>Ir HDR brachytherapy sources were determined experimentally. A radiation quality correction factor <jats:italic>k<jats:sub>Q</jats:sub></jats:italic> was determined as the ratio of the calibration coefficients for <jats:sup>60</jats:sup>Co and <jats:sup>192</jats:sup>Ir. The dependence on chamber‐to‐chamber variations, source‐to‐source variations, and source strength was investigated.</jats:p></jats:sec><jats:sec><jats:title>Results:</jats:title><jats:p>For the PTW T<jats:italic>x</jats:italic>33004 (Nucletron source dosimetry system (SDS)) well‐type chamber, the type‐specific radiation quality correction factor <jats:italic>k<jats:sub>Q</jats:sub></jats:italic> is 1.19. Note that this value is valid for chambers with the serial number, SN ≥ 315 (Nucletron SDS SN ≥ 548) onward only. For the Standard Imaging HDR 1000 Plus well‐type chambers, the type‐specific correction factor <jats:italic>k<jats:sub>Q</jats:sub></jats:italic> is 1.05. Both <jats:italic>k<jats:sub>Q</jats:sub></jats:italic> values are independent of the source strengths in the complete clinically relevant range. The relative expanded uncertainty (<jats:italic>k</jats:italic> = 2) of <jats:italic>k<jats:sub>Q</jats:sub></jats:italic> is <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/mp2684-math-0035.png" xlink:title="urn:x-wiley:00942405:mp2684:equation:mp2684-math-0035"/> = 2.1% for both chamber types.</jats:p></jats:sec><jats:sec><jats:title>Conclusions:</jats:title><jats:p>The calibration coefficient of a well‐type chamber for radiation fields of <jats:sup>60</jats:sup>Co HDR brachytherapy sources can be calculated from a given calibration coefficient for <jats:sup>192</jats:sup>Ir radiation by using a chamber‐type‐specific radiation quality correction factor <jats:italic>k<jats:sub>Q</jats:sub></jats:italic>. However, the uncertainty of a <jats:sup>60</jats:sup>Co calibration coefficient calculated via <jats:italic>k<jats:sub>Q</jats:sub></jats:italic> is at least twice as large as that for a direct calibration with a <jats:sup>60</jats:sup>Co source.</jats:p></jats:sec>