A radiation quality correction factor k Q for well‐type ionization chambers for the measurement of the reference air kerma rate of 60 Co HDR brachytherapy sources

Purpose: The aim of this study was to investigate whether a chamber‐type‐specific radiation quality correction factor k Q can be determined in order to measure the reference air kerma rate of 60 Co high‐dose‐rate (HDR) brachytherapy sources with acceptable uncertainty by means of a well‐type ionization chamber calibrated for 192 Ir HDR sources. Methods: The calibration coefficients of 35 well‐type ionization chambers of two different chamber types for radiation fields of 60 Co and 192 Ir HDR brachytherapy sources were determined experimentally. A radiation quality correction factor k Q was determined as the ratio of the calibration coefficients for 60 Co and 192 Ir. The dependence on chamber‐to‐chamber variations, source‐to‐source variations, and source strength was investigated. Results: For the PTW T x 33004 (Nucletron source dosimetry system (SDS)) well‐type chamber, the type‐specific radiation quality correction factor k Q 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 k Q is 1.05. Both k Q values are independent of the source strengths in the complete clinically relevant range. The relative expanded uncertainty ( k = 2) of k Q isU k Q= 2.1% for both chamber types. Conclusions: The calibration coefficient of a well‐type chamber for radiation fields of 60 Co HDR brachytherapy sources can be calculated from a given calibration coefficient for 192 Ir radiation by using a chamber‐type‐specific radiation quality correction factor k Q . However, the uncertainty of a 60 Co calibration coefficient calculated via k Q is at least twice as large as that for a direct calibration with a 60 Co source.