SUMMARY

There is growing evidence to support increasing the relative biological effectiveness (RBE) of low energy electrons and photons[1-3]. Studies on 250 kVp x-rays imply that this radiation is 2-3 times more efficient at producing dicentric chromosomes when compared to the gammas produced from 60Co. Furthermore, studies on x-rays below 30 kVp claim that the RBE is even higher than that of 250 kVp x-rays[4]. Strong justification for increasing the RBE of low energy electrons also comes from studies on the beta spectrum of 3H for which Kocher[5] reports a RBE of 2.4. 30 kVp These facts are particularly relevant because 30 kVp x-rays are used in mammography[6] while tritium constitutes the largest release term from nuclear power plants[7]. This increase in the RBE of low energy electron radiation can be explained by the fact that the lineal energy transfer of electrons rises steeply as the electron slows down and eventually comes to a rest. Since photons primarily deposit their energy indirectly through electron interactions, the increased RBE of 250kVp x-rays can also be accounted by the increased LET of low energy electrons. This research effort’s primary focus is to bridge the gaps between experimentally measured RBEs of low energy electrons and photons using Monte Carlo electron transport track analysis. In doing so, a general method of estimating the RBE of arbitrary low energy photon and electron sources will be developed.