Some of the physical parameters necessary in electron beam treatment planning are investigated for 5‐ to 45‐MeV electrons extracted from a Brown–Boveri betatron. The percent depth ionization (PDI) curves are compared with the empirical relation given by Laughlin's equation, and an extension of Laughlin's equation is described. The modified absorption coefficient (MAC) method is introduced to correct the isodose distributions for the presence of inhomogeneities, such as lung, with the actual density of lung and the location of the inhomogeneity taken into consideration. Experimental data from measurements made in a water‐cork phantom are presented, and the results are compared with the various calculated methods.