Challenges in Monte Carlo track structure modelling

Purpose: Although great progress has been made, numerous challenges remain in the development of Monte Carlo (MC) charged-particle track structure simulation models. Such models have evolved from the simple gas phase target models to those using condensed phase interaction data coupled with complex targets representing cellular and molecular constituents of mammalian tissue. A wide choice of MC models is now available ranging from those based on the physics of continuous slowing down, to simulations following each interaction on an event-by-event basis. The choice of code depends largely on requirements for computational speed, and the degree of detail required; however, one must be continuously vigilant to recognise the inherent limitations of the model chosen. Conclusions: There remain numerous questions of the accuracy and completeness of the interaction physics that present challenges to MC modellers. Recent evidence suggests that the yields of electrons with energies less than a few hundred eV might be substantially overestimated by the elastic and inelastic cross-sections used in many codes. Densely ionising heavy ions present modelling challenges when the rate of energy loss is sufficient to ionise essentially 'every' atom along the ion path. Effects of electron capture and loss by moving heavy ions present significant challenges for modellers particularly for accurate simulation for ions heavier than protons and helium ions? The average effective-charge provides an inadequate description for estimating differential cross-sections for energy loss. These and other questions are considered. (authors)