  We study energy deposition by light nuclei in tissue-like media taking into
account nuclear fragmentation reactions, in particular, production of secondary
neutrons. The calculations are carried out within a Monte Carlo model for
Heavy-Ion Therapy (MCHIT) based on the GEANT4 toolkit. Experimental data on
depth-dose distributions for 135A-400A MeV C-12 and O-18 beams are described
very well without any adjustment of the model parameters. This gives confidence
in successful use of the GEANT4 toolkit for MC simulations of cancer therapy
with beams of light nuclei. The energy deposition due to secondary neutrons
produced by C-12 and Ne-20 beams in a (40-50 cm)^3 water phantom is estimated
to 1-2% of the total dose, that is only slightly above the neutron contribution
(~1%) induced by a 200 MeV proton beam.
