A simple derivation of Ngas, a correction in Awall, and other comments on the AAPM Task Group 21 protocol

The new AAPM Task Group 21 dosimetry protocol is a useful document that will no doubt result in improved accuracy in radiotherapy beam standardization when it is generally understood and widely adopted. It does however, require some "debugging." The present paper clarifies the derivation of Ngas, points out an error in the Awall data, and discusses the impracticality of applying the gradient approach in correcting for ion chamber perturbation of dose in phantoms irradiated by electron beams.