  We model the responses of Tissue-Equivalent Proportional Counters (TEPC) to
radiation fields of therapeutic C-12 beams in a water phantom and to
quasi-monoenergetic neutrons in a PMMA phantom. Simulations are performed with
the Monte Carlo model for Heavy Ion Therapy (MCHIT) based on the Geant4
toolkit. The shapes of the calculated lineal energy spectra agree well with
measurements in both cases. The influence of fragmentation reactions on the
TEPC response to a narrow pencil-like beam with its width smaller than the TEPC
diameter is investigated by Monte Carlo modeling. It is found that total lineal
energy spectra are not very sensitive to the choice of the nuclear
fragmentation model used. The calculated frequency-mean lineal energy y_f
differs from the data on the axis of a therapeutic beam by less than 10% and by
10-20% at other TEPC positions. The validation of MCHIT with neutron beams
gives us confidence in estimating the contributions to lineal energy spectra
due to secondary neutrons produced in water by C-12 nuclei. As found, the
neutron contribution at 10 cm distance from the beam axis amounts to ~ 50%
close the entrance to the phantom and decreases to ~ 25% at the depth of the
Bragg peak and beyond it. The presented results can help in evaluating
biological out-of-field doses in carbon-ion therapy.
