Technical note: Determination of CQ$C_{Q}$ for a miniature x‐ray source using a soft x‐ray ionization chamber calibrated in NIST reference beam qualities

Background The CQ$C_Q$ formalism proposed by Watson et al. allows users of the INTRABEAM (Carl Zeiss Medical AG, Jena, Germany) electronic brachytherapy system to accurately determine the absorbed dose to water, in the absence of a primary dosimetry standard. However, all published CQ$C_Q$ values are for PTW 34013 ionization chambers calibrated in a TW30 reference beam, traceable to PTB (Germany). For North American users, it would be advantageous to have CQ$C_Q$ data for chambers calibrated in a kV reference beam maintained by the National Institute of Standards and Technology (NIST). Purpose In this work, we determine CQ$C_Q$ for a PTW 34013 chamber calibrated in three NIST‐traceable reference beams: M30, L40, and L50. Methods Using available photon spectra data for M30, L40, and L50 reference beam qualities, Monte Carlo simulations using EGSnrc were performed to calculate the ratio of the absorbed dose to the PTW 34013 chamber air cavity to air‐kerma (Dgas/Ka$D_{\textrm {gas}}/K_a$) for these beams. From this ratio, CQ$C_Q$ as a function of depth in water was determined. The effect of the use of a buildup foil was also investigated. An uncertainty analysis considering both the Type A and Type B uncertainties in the calculation of CQ$C_Q$ was performed. Results The largest difference in CQ$C_Q$ was found between L50 and TW30, with a relative decrease of 1.4% (no buildup) to 1.6% (buildup). For M30 and L40, the differences were minimal compared with measurement uncertainties. Conclusions We report CQ$C_Q$ values for three NIST‐traceable kV reference beams. This study reinforces the feasibility of adapting the Watson et al. methodology using different kV reference beams, facilitating the use of INTRABEAM in North America and ensuring the continuity and accuracy of dosimetry standards in intraoperative radiation therapy.