  A Geant4-based Monte Carlo model for Heavy-Ion Therapy (MCHIT) is used to
study radiation fields of H-1, He-4, Li-7 and C-12 beams with similar ranges
(~160-180 mm) in water. Microdosimetry spectra are simulated for wall-less and
walled Tissue Equivalent Proportional Counters (TEPCs) placed outside or inside
a phantom, as in experiments performed, respectively, at NIRS, Japan and GSI,
Germany. The impact of fragmentation reactions on microdosimetry spectra is
investigated for He-4, Li-7 and C-12, and contributions from nuclear fragments
of different charge are evaluated for various TEPC positions in the phantom.
The microdosimetry spectra measured on the beam axis are well described by
MCHIT, in particular, in the vicinity of the Bragg peak. However, the simulated
spectra for the walled TEPC far from the beam axis are underestimated. Relative
Biological Effectiveness (RBE) of the considered beams is estimated using a
modified microdosimetric-kinetic model. Calculations show a similar rise of the
RBE up to 2.2-2.9 close to the Bragg peak for helium, lithium and carbon beams
compared to the modest values of 1-1.2 at the plateau region. Our results
suggest that helium and lithium beams are also promising options for cancer
therapy.
