CALCULATIONS OF ELECTRON SINGLE EVENT DISTRIBUTIONS FOR USE IN INTERNAL BETA-MICRODOSIMETRY

In this paper, initial work in developing the microdosimetry for beta emitting radionuclides used in radioimmunotherapy (RIT) is described. First, a systematic tabulation of single event distributions f1(z) is made using the Monte Carlo electron transport code OREC developed at t he Oak Ridge National Laboratory. OREC considers each and every energy loss event within a liquid water medium, as well as elastic collision s which become prominent at low energies. Calculations of f1(z,E,d) ar e presented as functions of electron energy E and the target site diam eter d for point sources on the surface of a model cell. Since electro ns do not follow linear paths, electron hit probabilities w(E,d) must be obtained explicitly from the transport calculations. Additional inf ormation is provided as to the energy loss contributions made by (1) p rimary versus secondary electrons, and (2) ionisations versus excitati ons. Such information may prove useful in interpreting biological effe cts of beta particle RIT.