Microdosimetry of 12-20 MeV electrons

Experimental techniques have been developed for obtaining microdosimetric spectra from hospital-based linear accelerators. Teletherapy beams of 12, 15, 18, and 20 MeV electrons from Clinac 18 and 20 accelerators have been produced at ultra low dose rates. Details of the experimental methods have been described previously by the authors. Linear energy spectra for the beams, as measured by a Rossi chamber, differ significantly from Cobalt-60 both in shape and in the average linear energy (y-bar/sub d/) values which, depending on electron energy, can be 20-40% lower. The spectral peaks are also greatly enhanced for electrons. The y-bar/sub d/ for electrons also differs from values for 10-15 MeV photon beams, but these differences are less pronounced than the differences with Cobalt and are highly energy dependent. Spectral peaks and shapes can be explained if one considers that the actual electron stopping power in tissue reaches a minimum at 1.5 MeV. The theory of dual radiation action predicts changes in RBE at low doses for megavoltage photons and electrons proportional to these differences in y-bar/sub d/. The magnitude of this effect may even be significant at clinical doses