A Monte Carlo study of cellular S-factors for 1 keV to 1 MeV electrons

A systematic study of cellular S-factors and absorbed fractions for monoenergetic electrons of initial energy from 1 keV to 1 MeV is presented. The calculations are based on our in-house Monte Carlo codes which have been developed to simulate electron transport up to a few MeV using both event-by-event and condensed-history techniques. An extensive comparison with the MIRD tabulations is presented for spherical volumes of 1-10 microm radius and various source-to-target combinations relevant to the intracellular localization of the emitted electrons. When the primary electron range is comparable to the sphere radius, we find significantly higher values from the MIRD, while with increasing electron energy the escape of delta-rays leads gradually to the opposite effect. The largest differences with the MIRD are found for geometries where the target region is at some distance from the source region (e.g. surface-to-nucleus or cytoplasm-to-nucleus). The sensitivity of the results to different transport approximations is examined. The grouping of inelastic collisions is found adequate as long as delta-rays are explicitly simulated, while the inclusion of straggling for soft collisions has a negligible effect.