Subcellular S-factors for low-energy electrons: a comparison of Monte Carlo simulations and continuous-slowing-down calculations.

Purpose: To study the energy deposition by low-energy electrons in submicron tissue-equivalent targets by comparing two widely used methodologies, namely, the continuous-slowing-down-approximation (CSDA) convolution integral and the Monte Carlo (MC) simulation.Methods: An MC track-structure code that simulates collision-by-collision the complete slowing down process is used to calculate the energy deposition in spherical volumes of unit density water medium. Comparisons are made with calculations based on the CSDA convolution integral using both empirical and MC-based range-energy analytic formulae.Results: We present self-irradiation absorbed fractions and S-factors for monoenergetic electrons of initial energies from 0.1–10 keV distributed uniformly in spheres of 5, 10, 50, 100, 500, and 1000 nm radius. The MC and CSDA results were found, in some cases, to differ by a factor of 2 or more; differences generally increase with decreasing sphere size. Contrary to high energies, the uncertainties associated ...