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

Abstract

We model the responses of Tissue-Equivalent Proportional Counters (TEPC) to radiation fields of therapeutic 12 C beams in a water phantom and to quasimonoenergetic 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 energyȳ 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 12 C 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.

Microdosimetry spectra and RBE of 1H, 4He, 7Li and 12C nuclei in water studied with Geant4

“…As demonstrated recently, MCHIT describes well microdosimetry spectra for neutron and carbon-ion beams [18]. In this work we present Monte Carlo calculations of microdosimetry distributions for proton, 4 He, 7 Li and 12 C beams in water and compare results with experimental data.…”

“…Similar to the calculations for 1 H and 4 He beams described above, the TEPC positions were adjusted in order to reproduce the position of the main peak in the spectra when the TEPCs were placed on the axis in the vicinity of the Bragg peak of 7 Li and 12 C beams. The shift of the TEPC for the carbon beam (2 mm deeper position which corresponds to 1% of the range of carbon ion) was discussed elsewhere [18]. As for 7 Li beam, the microdosimetry spectrum measured for the TEPC located at the Bragg peak could be only reproduced by calculations if the TEPC is further shifted to deeper position by 6.7 mm.…”

“…11. The exact TEPC positions used in simulations are given in our previous publication [18], where further details on the calculational procedure can be found. A shoulder in the spectrum at low y at "0 cm, plateau" position is due to the contribution of several projectile fragments.…”

First theoretical determination of relative biological effectiveness of very high energy electrons

“…Error bars correspond to the standard error σ i associated to each bin content y i of the calculated spectra f(y) , propagated through the calculation of the yd(y) spectra (see section " Radiobiological calculations with MKM "). The choice of characteristic depths for ion beams (plateau, peak, distal) is made according to their relevance in observing the largest evolution of microscopic characteristics along the beam path, in coherence with other microdosimetric studies 36 , 37 . The characteristic depths for electron and photon beams (R100, R85, R50) were chosen to compare the different beams over their entire range because of their relevance in clinical dosimetry, although it corresponds to very different absolute depths in water according to the electron energy used.…”

Accurate simulations of TEPC neutron spectra using Geant4

“…This is because the energy of such delta rays lies below the default lower limit in QGSP_BERT_HP of 0.99 keV on the production threshold for secondaries. Burigo et al (2013) discuss in detail the effect of such thresholds in a TEPC context. In the present study, however, it is not thought likely that this threshold could account for the difference with experiment.…”