Monte Carlo approaches to important problems in radiotherapy dosimetry

Monte Carlo codes such as EGS4 and MCNP are essential for simulating electron and photon transport in radiotherapy, e.g. Monte Carlo (MC) derived values of the water-to-air stopping-power ratio, S-w/air, are used in all Codes of Practice for absolute dose determination in radiotherapy clinics. Further, MC simulation can compute directly D-med/D-det for a dosimeter. Low gas density in ionisation chambers poses problems for most codes based on condensed history (CH) electron transport. However, combining single- with multiple-scattering, as recently implemented in EGSnrc, enables arbitrarily accurate chamber-response simulations. Monte Carlo can now compute doses in radiotherapy patients with uncertainties of approximate to1% (1 SD) in mm-sized voxels in a few minutes for an electron beam and in <60 min. for photon beams using sub $20000 hardware, thus avoiding approximations previously made for the effect of heterogeneities. In the microdosimetry/track structure field, CH based codes demonstrate depth- and (incident) energy-independent electron (dPhi/dE) per unit dose at low energies, thus explaining the negligible RBE variation over a wide range of low-LET radiations.