Microdosimetry of radiation field from a therapeutic 12C beam in water: A study with Geant4 toolkit

We model the responses of Tissue-Equivalent Proportional Counters (TEPC) to radiation fields of therapeutic 12C beam 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 carbon beam with its width smaller than the TEPC diameter is investigated by Monte Carlo modeling. It is found that the total lineal energy spectra are not very sensitive to the choice of the nuclear fragmentation model used by MCHIT. The calculated frequency-mean lineal energy differs from the data on the axis of a therapeutic beam by less than 10% and by 10–20% at other TEPC positions. Following the validation of MCHIT with quasi-monoenergetic neutron beams the contributions to lineal energy spectra from secondary neutrons produced by 12C nuclei in water are estimated. As found, the neutron contribution at 10 cm distance from the beam axis amounts to ∼50% close to 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.