Monte Carlo Model For Heavy-Ion Therapy Research Articles

Distributions of deposited energy and ionization clusters around ion tracks studied with Geant4 toolkit

The Geant4-based Monte Carlo model for Heavy-Ion Therapy (MCHIT) was extended to study the patterns of energy deposition at sub-micrometer distance from individual ion tracks. Dose distributions for low-energy 1H, 4He, 12C and 16O ions measured in several experiments are well described by the model in a broad range of radial distances, from 0.5 to 3000 nm. Despite the fact that such distributions are characterized by long tails, a dominant fraction of deposited energy (∼80%) is confined within a radius of about 10 nm. The probability distributions of clustered ionization events in nanoscale volumes of water traversed by 1H, 2H, 4He, 6Li, 7Li, and 12C ions are also calculated. A good agreement of calculated ionization cluster-size distributions with the corresponding experimental data suggests that the extended MCHIT can be used to characterize stochastic processes of energy deposition to sensitive cellular structures.

Comparative study of dose distributions and cell survival fractions for 1H, 4He, 12C and 16O beams using Geant4 and Microdosimetric Kinetic model

Depth and radial dose profiles for therapeutic 1H, 4He, 12C and 16O beams are calculated using the Geant4-based Monte Carlo model for Heavy-Ion Therapy (MCHIT). 4He and 16O ions are presented as alternative options to 1H and 12C broadly used for ion-beam cancer therapy. Biological dose profiles and survival fractions of cells are estimated using the modified Microdosimetric Kinetic model. Depth distributions of cell survival of healthy tissues, assuming 10% and 50% survival of tumor cells, are calculated for 6 cm SOBPs at two tumor depths and for different tissues radiosensitivities. It is found that the optimal ion choice depends on (i) depth of the tumor, (ii) dose levels and (iii) the contrast of radiosensitivities of tumor and surrounding healthy tissues. Our results indicate that 12C and 16O ions are more appropriate to spare healthy tissues in the case of a more radioresistant tumor at moderate depths. On the other hand, a sensitive tumor surrounded by more resistant tissues can be better treated with 1H and 4He ions. In general, 4He beam is found to be a good candidate for therapy. It better spares healthy tissues in all considered cases compared to 1H. Besides, the dose conformation is improved for deep-seated tumors compared to 1H, and the damage to surrounding healthy tissues is reduced compared to heavier ions due to the lower impact of nuclear fragmentation. No definite advantages of 16O with respect to 12C ions are found in this study.

Radiation quality of cosmic ray nuclei studied with Geant4-based simulations

In future missions in deep space a space craft will be exposed to a non-negligible flux of high charge and energy (HZE) particles present in the galactic cosmic rays (GCR). One of the major concerns of manned missions is the impact on humans of complex radiation fields which result from the interactions of HZE particles with the spacecraft materials. The radiation quality of several ions representing GCR is investigated by calculating microdosimetry spectra. A Geant4-based Monte Carlo model for Heavy Ion Therapy (MCHIT) is used to simulate microdosimetry data for HZE particles in extended media where fragmentation reactions play a certain role. Our model is able to reproduce measured microdosimetry spectra for H, He, Li, C and Si in the energy range of 150-490 MeV/u. The effect of nuclear fragmentation on the relative biological effectiveness (RBE) of He, Li and C is estimated and found to be below 10%.

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

A Geant4-based Monte Carlo model for Heavy-Ion Therapy (MCHIT) is used to study radiation fields of 1H, 4He, 7Li and 12C beams with similar ranges (∼160–180mm) in water. Microdosimetry spectra are simulated for wall-less and walled Tissue Equivalent Proportional Counters (TEPCs) placed outside or inside a phantom, as in experiments performed, respectively, at NIRS, Japan and GSI, Germany. The impact of fragmentation reactions on microdosimetry spectra is investigated for 4He, 7Li and 12C, and contributions from nuclear fragments of different charge are evaluated for various TEPC positions in the phantom. The microdosimetry spectra measured on the beam axis are well described by MCHIT, in particular, in the vicinity of the Bragg peak. However, the simulated spectra for the walled TEPC far from the beam axis are underestimated. Relative Biological Effectiveness (RBE) of the considered beams is estimated using a modified microdosimetric-kinetic model. Calculations show a similar rise of the RBE up to 2.2–2.9 close to the Bragg peak for helium, lithium and carbon beams compared to the modest values of 1–1.2 at the plateau region. Our results suggest that helium and lithium beams are also promising options for cancer therapy.

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 10cm 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.

Modelling heavy-ion energy deposition in extended media

We present recent developments of the Monte Carlo model for heavy-ion therapy (MCHIT), which is currently based on the Geant4 toolkit of version 9.2. The major advancement of the model concerns the modelling of violent fragmentation reactions by means of the Fermi break-up model, which is used to simulate decays of hot fragments created after the first stage of nucleus-nucleus collisions. By means of MCHIT we study the dose distributions from therapeutic beams of carbon nuclei in tissue-like materials, like water and PMMA. The contributions to the total dose from primary beam nuclei and from charged secondary fragments produced in nuclear fragmentation reactions are calculated. The build-up of secondary fragments along the beam axis is calculated and compared with available experimental data. Finally, we demonstrate the impact of violent multifragment decays on energy distributions of secondary neutrons produced by carbon nuclei in water.

PET monitoring of cancer therapy with 3He and 12C beams: a study with the GEANT4 toolkit

We study the spatial distributions of β+-activity produced by therapeutic beams of 3He and 12C ions in various tissue-like materials. The calculations were performed within a Monte Carlo model for heavy-ion therapy (MCHIT) based on the GEANT4 toolkit. The contributions from positron-emitting nuclei with T1/2 > 10 s, namely 10,11C, 13N, 14,15O, 17,18F and 30P, were calculated and compared with experimental data obtained during and after irradiation, where available. Positron-emitting nuclei are created by a 12C beam in fragmentation reactions of projectile and target nuclei. This leads to a β+-activity profile characterized by a noticeable peak located close to the Bragg peak in the corresponding depth–dose distribution. This can be used for dose monitoring in carbon-ion therapy of cancer. In contrast, as most of the positron-emitting nuclei are produced by a 3He beam in target fragmentation reactions, the calculated total β+-activity during or soon after the irradiation period is evenly distributed within the projectile range. However, we predict also the presence of 13N, 14O, 17,18F created in charge-transfer reactions by low-energy 3He ions close to the end of their range in several tissue-like media. The time evolution of β+-activity profiles was investigated for both kinds of beams. We found that due to the production of 18F nuclides the β+-activity profile measured 2 or 3 h after irradiation with 3He ions will have a distinct peak correlated with the maximum of depth–dose distribution. We also found certain advantages of low-energy 3He beams over low-energy proton beams for reliable PET monitoring during particle therapy of shallow-located tumours. In this case the distal edge of β+-activity distribution from 17F nuclei clearly marks the range of 3He in tissues.

Distributions of positron-emitting nuclei in proton and carbon-ion therapy studied with GEANT4

Depth distributions of positron-emitting nuclei in PMMA phantoms are calculated within a Monte Carlo model for heavy-ion therapy (MCHIT) based on the GEANT4 toolkit (version 8.0). The calculated total production rates of 11C, 10C and 15O nuclei are compared with experimental data and with corresponding results of the FLUKA and POSGEN codes. The distributions of e+ annihilation points are obtained by simulating radioactive decay of unstable nuclei and transporting positrons in the surrounding medium. A finite spatial resolution of the positron emission tomography (PET) is taken into account in a simplified way. Depth distributions of β+-activity as seen by a PET scanner are calculated and compared to available data for PMMA phantoms. The obtained β+-activity profiles are in good agreement with PET data for proton and 12C beams at energies suitable for particle therapy. The MCHIT capability to predict the β+-activity and dose distributions in tissue-like materials of different chemical composition is demonstrated.For more information on this article, see medicalphysicsweb.org

Neutrons from fragmentation of light nuclei in tissue-like media: a study with the GEANT4 toolkit

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 12C and 18O 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 12C and 20Ne 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 (∼1%) induced by a 200 MeV proton beam.