Restricted LET investigations around high-energy electron beams

Abstract

Reports on the measurement of the restricted dose mean LET with a cut-off energy Delta =500 eV (L500,D) a few centimetres outside the field edge of a clinical electron accelerator for 6-20 MeV electrons as a function of field size. The results indicate that the L500,D of the scattered radiation a few centimetres outside the field of view is not significantly increased. Despite the multiple scattering that takes place at these points it seems that most of the dose still originates from electrons in the MeV energy region and thus little differences occur in the restricted LET spectrum, as restricted LET is a slow function of electron energy at high energies (ICRU 1970). This result is in contrast with what has been found for 60Co, where a 50% increase in the L500,D has been measured outside the field edge (Makrigiorgos and Antonadou, 1987). Thus, in the case of clinical high-energy electrons scattered from the main beam, no increase in the relative biological effectiveness due to restricted LET differences is expected in the region a few centimetres outside the treatment area.