The mass angular scattering power method for determining the kinetic energies of clinical electron beams

A method for determining the kinetic energy of clinical electron beams is described. The method is based on the measurement in air of the spatial spread of a pencil electron beam which is produced from the broad clinical electron beam. As predicted by the Fermi-Eyges theory, the dose distribution measured in air on a plane, perpendicular to the incident direction of the initial pencil electron beam, is Gaussian. The square of its spatial spread is related to the mass angular scattering power which in turn is related to the kinetic energy of the electron beam. The measured spatial spread may thus be used to determine the mass angular scattering power, which is then used to determine the kinetic energy of the electron beam from the known relationship between mass angular scattering power and kinetic energy. Energies obtained with the mass angular scattering power method agree with those obtained with the electron range method. The angular scattering power method is relatively cumbersome, but allows us to determine the kinetic energies of electron beams from first principles, in contrast to the empirical methods based on range measurements in water.