Monte Carlo simulation of a TEPC for microdosimetry of carbon ions

The increase in the number of therapeutic proton and ion beam centres worldwide has prompted renewed interest in measuring and simulating microdosimetric spectra in order to help understand the complexity underlying the Relative Biological Effectiveness (RBE) of these treatment modalities. In this context we have studied the capability of the Geant4 toolkit to simulate microdosimetric spectra measured with a Tissue Equivalent Proportional Counter (TEPC) in a clinical carbon ion beam. The simulated spectra were compared with published experimental data obtained along the depth dose curve of a 194 MeV/u carbon beam at the GSI, Darmstadt (Gerlach et al., 2002). The initial beam energy and energy spread employed in the simulation were tuned to match the calculated and measured depth dose distributions. A good agreement was found at all depths after a shift of 4.025 mm was taken into account with agreement for the microdosimetric derived RBE values to within 0.4% and 11.9% for depths 40 and 66 mm in PMMA (Perspex). This work demonstrates that the Geant4 toolkit can accurately reproduce experimental microdosimetric data and can thus be used for independent calculation of lineal energy spectra from which RBE estimates can be derived using the equation of Pihet et al. (1990). The work highlights the difficulty in using experimental work to benchmark Monte Carlo simulations and the need for detailed descriptions of experimental setups used.