  We study the propagation of nucleons and nuclei in tissue-like media within a
Monte Carlo Model for Heavy-ion Therapy (MCHIT) based on the GEANT4 toolkit
(version 8.2). The model takes into account fragmentation of projectile nuclei
and secondary interactions of produced nuclear fragments. Model predictions are
validated with available experimental data obtained for water and PMMA phantoms
irradiated by monoenergetic carbon-ion beams. The MCHIT model describes well
(1) the depth-dose distributions in water and PMMA, (2) the doses measured for
fragments of certain charge, (3) the distributions of positron emitting nuclear
fragments produced by carbon-ion beams, and (4) the energy spectra of secondary
neutrons measured at different angles to the beam direction. Radial dose
profiles for primary nuclei and for different projectile fragments are
calculated and discussed as possible input for evaluation of biological dose
distributions. It is shown that at the periphery of the transverse dose profile
close to the Bragg peak the dose from secondary nuclear fragments is comparable
to the dose from primary nuclei.
