AAPM TG-51 and electron beam quality

In AAPM TG-51 protocol, the electron beam quality is specified by the beam penetration parameter, R50,D which is the depth where the electron beam depth dose drops to 50% of the peak value. This is more accurate than the parameter, Eo, the mean energy of the electron beam used in the earlier protocols (as derived from R50 using an empirical equation)