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 135-400 A MeV (12)C and (18)O 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 (12)C and (20)Ne beams in a (40-50 cm)(3) water phantom is estimated to be 1-2% of the total dose, that is only slightly above the neutron contribution (approximately 1%) induced by a 200 MeV proton beam.