Fluence calculation methods in Monte Carlo dosimetry simulations

Detailed information on the different methods used to compute charged-particle fluence spectra during a Monte Carlo (MC) calculation is scattered throughout the literature or discussed in internal reports. This work summarizes the most commonly used methods and introduces an alternative approach, makes comparisons between the different techniques, both from a theoretical ground and performing ad-hoc MC calculations, and discusses the advantages and constraints of each technique. It is concluded that methods based on the apportion of a track segment to the different energy bins of a linear or logarithmic grid are independent of the length of the track segment and the amount of energy loss between its extremes. This is the case for two of the methods presented, but not for a third one, the former group being considered to yield more accurate distributions in most cases. It is shown that the positron fluence contribution to the total restricted cema may amount up to several percent, and its omission lead to cema underestimates of that order. The influence of restricted radiative energy losses of electrons and positrons on fluence distribution and cema calculations are discussed on the grounds of the relative weight of restricted and unrestricted stopping powers, leading to expect a practically negligible influence on dosimetry calculations. The expectation is confirmed with MC calculations of a high-energy photon beam in gold, leading to the conclusion that restricted radiative energy losses can be disregarded for the most commonly used threshold energies for secondary charged particle production.