Irena Gudowska

Ion beam transport in tissue-like media using the Monte Carlo code SHIELD-HIT

The development of the Monte Carlo code SHIELD-HIT (heavy ion transport) for the simulation of the transport of protons and heavier ions in tissue-like media is described. The code SHIELD-HIT, a spin-off of SHIELD (available as RSICC CCC-667), extends the transport of hadron cascades from standard targets to that of ions in arbitrary tissue-like materials, taking into account ionization energy-loss straggling and multiple Coulomb scattering effects. The consistency of the results obtained with SHIELD-HIT has been verified against experimental data and other existing Monte Carlo codes (PTRAN, PETRA), as well as with deterministic models for ion transport, comparing depth distributions of energy deposition by protons, 12C and 20Ne ions impinging on water. The SHIELD-HIT code yields distributions consistent with a proper treatment of nuclear inelastic collisions. Energy depositions up to and well beyond the Bragg peak due to nuclear fragmentations are well predicted. Satisfactory agreement is also found with experimental determinations of the number of fragments of a given type, as a function of depth in water, produced by 12C and 14N ions of 670 MeV u(-1), although less favourable agreement is observed for heavier projectiles such as 16O ions of the same energy. The calculated neutron spectra differential in energy and angle produced in a mimic of a Martian rock by irradiation with 12C ions of 290 MeV u(-1) also shows good agreement with experimental data. It is concluded that a careful analysis of stopping power data for different tissues is necessary for radiation therapy applications, since an incorrect estimation of the position of the Bragg peak might lead to a significant deviation from the prescribed dose in small target volumes. The results presented in this study indicate the usefulness of the SHIELD-HIT code for Monte Carlo simulations in the field of light ion radiation therapy.

Benchmarking nuclear models of FLUKA and GEANT4 for carbon ion therapy

As carbon ions, at therapeutic energies, penetrate tissue, they undergo inelastic nuclear reactions and give rise to significant yields of secondary fragment fluences. Therefore, an accurate prediction of these fluences resulting from the primary carbon interactions is necessary in the patients body in order to precisely simulate the spatial dose distribution and the resulting biological effect. In this paper, the performance of nuclear fragmentation models of the Monte Carlo transport codes, FLUKA and GEANT4, in tissue-like media and for an energy regime relevant for therapeutic carbon ions is investigated. The ability of these Monte Carlo codes to reproduce experimental data of charge-changing cross sections and integral and differential yields of secondary charged fragments is evaluated. For the fragment yields, the main focus is on the consideration of experimental approximations and uncertainties such as the energy measurement by time-of-flight. For GEANT4, the hadronic models G4BinaryLightIonReaction and G4QMD are benchmarked together with some recently enhanced de-excitation models. For non-differential quantities, discrepancies of some tens of percent are found for both codes. For differential quantities, even larger deviations are found. Implications of these findings for the therapeutic use of carbon ions are discussed.

Depth absorbed dose and LET distributions of therapeutic 1H, 4He, 7Li, and 12C beams.

The depth absorbed dose and LET (linear energy transfer) distribution of different ions of clinical interest such as 1H, 4He, 7Li, and 12C ions have been investigated using the Monte Carlo code SHIELD-HIT. The energies of the projectiles correspond to ranges in water and soft tissue of approximately 260 mm. The depth dose distributions of the primary particles and their secondaries have been calculated and separated with regard to their low and high LET components. A LET value below 10 eV/nm can generally be regarded as low LET and sparsely ionizing like electrons and photons. The high LET region may be assumed to start at 20 eV/nm where on average two double-strand breaks can be formed when crossing the periphery of a nucleosome, even though strictly speaking the LET limits are not sharp and ought to vary with the charge and mass of the ion. At the Bragg peak of a monoenergetic high energy proton beam, less than 3% of the total absorbed dose is comprised of high LET components above 20 eV/nm. The high LET contribution to the total absorbed dose in the Bragg peak is significantly larger with increasing ion charge as a natural result of higher stopping power and lower range straggling. The fact that the range straggling and multiple scattering are reduced by half from hydrogen to helium increases the possibility to accurately deposit only the high LET component in the tumor with negligible dose to organs at risk. Therefore, the lateral penumbra is significantly improved and the higher dose gradients of 7Li and 12C ions both longitudinally and laterally will be of major advantage in biological optimized radiation therapy. With increasing charge of the ion, the high LET absorbed dose in the beam entrance and the plateau regions where healthy normal tissues are generally located is also increased. The dose distribution of the high LET components in the 7Li beam is only located around the Bragg peak, characterized by a Gaussian-type distribution. Furthermore, the secondary particles produced by high energy 7Li ions in tissuelike media have mainly low LET character both in front of and beyond the Bragg peak.

Influence of multiple scattering and energy loss straggling on the absorbed dose distributions of therapeutic light ion beams: I. Analytical pencil beam model

The lateral and longitudinal distributions of absorbed dose of broad and narrow light ion beams in water are investigated. An analytical algorithm based on the generalized Fermi-Eyges theory is developed and used to calculate the effects of multiple scattering and range straggling on the dose distribution of light ion beams in water. A first-order Gaussian multiple scattering and energy loss straggling approach is generally sufficiently accurate for describing the lateral and longitudinal spread of the Bragg peak and the associated energy deposition distribution of therapeutic light ion beams at ranges of clinical interest. Nuclear reactions are not taken into account in this study. The analytical algorithm given in the present study allows an accurate description of the radial spread and the range straggling of light ions traversing matter. A verification of this approach by comparing with experimental data, Monte Carlo methods and other analytical techniques will be presented in a forthcoming paper.

An analytical model for light ion pencil beam dose distributions: multiple scattering of primary and secondary ions.

An analytical algorithm based on the generalized Fermi-Eyges theory, amended for multiple Coulomb scattering and energy loss straggling, is used for calculation of the dose distribution of light ion beams in water. Pencil beam energy deposition distributions are derived for light ions by weighting a Monte Carlo (MC) calculated planar integral dose distribution with analytically calculated multiple scattering and range straggling distributions. The planar integral dose distributions are calculated using the MC code SHIELD-HIT07, in which multiple scattering and energy loss straggling processes are excluded. The contribution from nuclear reactions is included in the MC calculations. Multiple scattering processes are calculated separately for primary and secondary ions and parameters of the initial angular and radial spreads, and the covariance of these are derived by a least-square parameterization of the SHIELD-HIT07 data. The results from this analytical algorithm are compared to pencil beam dose distributions obtained from SHIELD-HIT07, where all processes are included, as well as to experimental data. The presented analytical approach allows for the accurate calculation of the spatial energy deposition distributions of ions of atomic numbers Z = 1 - 8.

Simulations of microdosimetric quantities with the Monte Carlo code FLUKA for carbon ions at therapeutic energies

Abstract Purpose: Microdosimetric quantities can be used to estimate the biological effectiveness of radiation fields. This study evaluates the capability of the general-purpose Monte Carlo code FLUKA to simulate microscopic patterns of energy depositions for mixed radiation fields which are created by carbon ions at therapeutic energies in phantoms. Materials and methods: Measured lineal energy spectra and linear energy transfer (LET) spectra produced by carbon ions of about 300 MeV/n at different depths in phantoms representing human tissue were chosen from published literature and were compared with results from simulations of the measurement set-ups with FLUKA. Results: Simulations of the dose-weighted lineal energy spectra yd(y) and dose-weighted LET spectra describe the main features of the respective measured spectra. All simulated frequency mean and dose mean lineal energy values are, respectively, within 21% and 11% of the measured ones. A slight underestimation of fragment fluences is notable. It is shown that the simultaneous detection of several charged fragments in the TEPC (‘V effect’) has considerable impact on the measured lineal energy spectra of fragments. Conclusions: Agreement between measurements and FLUKA results is encouraging and shows that FLUKA can predict microdosimetric spectra of mixed radiation fields created by therapeutic carbon ions in phantoms reasonably well.

Geant4 Monte Carlo Simulations of the Belt Proton Radiation Environment On Board the International Space Station/Columbus

A detailed characterization of the trapped-proton-induced radiation environment on board Columbus and the International Space Station (ISS) has been carried out using the Geant4 Monte Carlo particle transport toolkit. Dose and dose equivalent rates, as well as penetrating particle spectra are presented. These results are based on detailed Geant4 geometry models of Columbus and ISS, comprising a total of about 1000 geometry volumes. Simulated trapped-proton dose rates are found to be strongly dependent on ISS altitude. Dose rates for different locations inside the Columbus cabin are presented, as well as for different models of the incident trapped-proton flux. Dose rates resulting from incident anisotropic trapped protons are found to be lower than, or equal to, those of omnidirectional models. The anisotropy induced by the asymmetric shielding distribution of Columbus/ISS is also studied. The simulated trapped-proton dose (equivalent) rates, averaged over different locations inside Columbus, are 120 muGy/d (154 muSv/d) and 79 muGy/d (102 muSv/d) for solar minimum and maximum conditions according to AP8 incident proton spectra and an ISS orbit of 380 km. The solar maximum dose rates are found to be of the same order as measurements in other modules in the present ISS.

FLUKA simulations of the response of tissue-equivalent proportional counters to ion beams for applications in hadron therapy and space

For both cancer therapy with protons and ions (hadron therapy) and space radiation environments, the spatial energy deposition patterns of the radiation fields are of importance for quantifying the resulting radiation damage in biological structures. Tissue-equivalent proportional counters (TEPC) are the principal instruments for measuring imparted energy on a microscopic scale and for characterizing energy deposition patterns of radiation. Moreover, the distribution of imparted energy can serve as a complementary quantity to particle fluences of the primary beam and secondary fragments for characterizing a radiation field on a physical basis for radiobiological models. In this work, the Monte Carlo particle transport code FLUKA is used for simulating energy depositions in TEPC by ion beams. The capability of FLUKA in predicting imparted energy and derived quantities, such as lineal energy, for microscopic volumes is evaluated by comparing it with a large set of TEPC measurements for different ion beams with atomic numbers ranging from 1 to 26 and energies from 80 up to 1000 MeV/n. The influence of different physics configurations in the simulation is also discussed. It is demonstrated that FLUKA can simulate energy deposition patterns of ions in TEPC cavities accurately and that it provides an adequate description of the main features of the spectra.

Geant4 Monte Carlo Simulations of the Galactic Cosmic Ray Radiation Environment On-Board the International Space Station/Columbus

A characterization of the Galactic cosmic ray (GCR) induced radiation environment on-board Columbus and the International Space Station (ISS) has been carried out using the Geant4 Monte Carlo particle transport toolkit and detailed geometry models of Columbus and ISS. Dose and dose equivalent rates, as well as penetrating particle spectra are presented. Simulation results indicate that the major part of the dose rates due to GCR protons are associated with secondary particles produced in the hull of ISS. Neutrons contribute about 15% of the GCR proton dose equivalent rate and mesons about 10%. More than 40% of the simulated GCR proton dose and dose equivalent rates are due to protons in the energy range above 10 GeV. Protons in the energy range above 50 GeV contribute only 5% to the dose rates. The total simulated dose and dose equivalent rates at solar maximum are 63 muGy/d and 123 muSv/d, respectively. The dose equivalent rate underestimates measurements made during the 2001 solar maximum. The discrepancy can be attributed to deficiencies in hadronic ion-nuclei interaction models for heavy ions and to the lack of such models above 10 GeV/N in Geant4.

Design of a fast multileaf collimator for radiobiological optimized IMRT with scanned beams of photons, electrons, and light ions

Intensity modulated radiation therapy is rapidly becoming the treatment of choice for most tumors with respect to minimizing damage to the normal tissues and maximizing tumor control. Today, intensity modulated beams are most commonly delivered using segmental multileaf collimation, although an increasing number of radiation therapy departments are employing dynamic multileaf collimation. The irradiation time using dynamic multileaf collimation depends strongly on the nature of the desired dose distribution, and it is difficult to reduce this time to less than the sum of the irradiation times for all individual peak heights using dynamic leaf collimation [Svensson et al., Phys. Med. Biol. 39, 37-61 (1994)]. Therefore, the intensity modulation will considerably increase the total treatment time. A more cost-effective procedure for rapid intensity modulation is using narrow scanned photon, electron, and light ion beams in combination with fast multileaf collimator penumbra trimming. With this approach, the irradiation time is largely independent of the complexity of the desired intensity distribution and, in the case of photon beams, may even be shorter than with uniform beams. The intensity modulation is achieved primarily by scanning of a narrow elementary photon pencil beam generated by directing a narrow well focused high energy electron beam onto a thin bremsstrahlung target. In the present study, the design of a fast low-weight multileaf collimator that is capable of further sharpening the penumbra at the edge of the elementary scanned beam has been simulated, in order to minimize the dose or radiation response of healthy tissues. In the case of photon beams, such a multileaf collimator can be placed relatively close to the bremsstrahlung target to minimize its size. It can also be flat and thin, i.e., only 15-25 mm thick in the direction of the beam with edges made of tungsten or preferably osmium to optimize the sharpening of the penumbra. The low height of the collimator will minimize edge scatter from glancing incidence. The major portions of the collimator leafs can then be made of steel or even aluminum, so that the total weight of the multileaf collimator will be as low as 10 kg, which may even allow high-speed collimation in real time in synchrony with organ movements. To demonstrate the efficiency of this collimator design in combination with pencil beam scanning, optimal radiobiological treatments of an advanced cervix cancer were simulated. Different geometrical collimator designs were tested for bremsstrahlung, electron, and light ion beams. With a 10 mm half-width elementary scanned photon beam and a steel collimator with tungsten edges, it was possible to make as effective treatments as obtained with intensity modulated beams of full resolution, i.e., here 5 mm resolution in the fluence map. In combination with narrow pencil beam scanning, such a collimator may provide ideal delivery of photons, electrons, or light ions for radiation therapy synchronized to breathing and other organ motions. These high-energy photon and light ion beams may allow three-dimensional in vivo verification of delivery and thereby clinical implementation of the BioArt approach using Biologically Optimized three-dimensional in vivo predictive Assay based adaptive Radiation Therapy [Brahme, Acta Oncol. 42, 123-126 (2003)].