Equations for Ngas and Nair in terms of Nx and Nk

Task Group 21 of the American Association of Physicists in Medicine defined the cavity-gas calibration factor Ngas for ionization chambers in a protocol for radiotherapy dosimetry published in this journal [Med. Phys. 10, 741 (1983)]. Later, Schulz et al., [Med. Phys. 13, 755 (1986)] published a letter of clarification that proposed a revised equation for Ngas in which the effect of atmospheric humidity was specifically dealt with. The present report points out errors in both of those presentations of the Ngas equation, and derives formulations for cases where the standardization laboratory does or does not correct for ambient humidity during chamber calibration. In the case where humidity is corrected for, the resulting quantity is more properly called "Nair." The Ngas equation is stated in terms of the air kerma calibration factor Nk as well as the more familiar exposure calibration factor Nx.