The mathematical anthropomorphic phantoms EVA-HIT and ADAM-HIT have been used in the Monte Carlo code SHIELD-HIT07 for simulations of lung tumor and prostate irradiation with light ions. Calculations were performed for ¹H, ⁷Li, and ¹²C beams of energies in the range of 80 to 330 MeV/u. The secondary doses to organs, due to scattered primary ions and secondary particles produced in the phantoms, were studied taking into account the contribution from secondary neutrons, secondary protons, pions, and heavier fragments from helium to calcium. The doses to organs per dose to target (tumor) are of the order of 10^-6 to 10^-1 mGy Gy^-1 and decrease with increasing distance from the target. In general the organ dose per target dose increases with increasing Z of the primary particle; however, for lighter primary ions (Z 3) and for organs close to the target, scattered primary particles show a nonnegligible dose contribution.

Secondary radiation exposure of patients undergoing radiation therapy with light ions is of great concern due to possible tissue damage and risk of induction of secondary cancers. Secondary particles such as neutrons, protons and heavier ions are produced when the primary ions interact through nuclear inelastic reactions with the beam-line components, and with the tissues of the patient. Evaluations of secondary doses delivered to an anthropomorphic male phantom under prostate irradiation with (1)H and (12)C ion beams with energies 172 MeV and 330 MeV/u, respectively, have been performed with the Monte Carlo code SHIELD-HIT. Fluences of secondary particles with atomic mass A = 1-7 and energies up to 200-600 MeV/u are observed in organs even at larger distances (40-50 cm) from the irradiated volume. The secondary absorbed doses in selected organs are discussed taking into account the dose contribution from secondary neutrons, and the contribution from charged fragments that are not the products of neutron interactions. For (12)C ion irradiation, a substantial contribution to the absorbed organ dose is due to charged fragments. This contribution decreases from 81% in the organs close to the irradiated volume to 35-40% in the organs at larger distances.

Secondary organ absorbed doses were calculated by Monte Carlo simulations with the SHIELD-HIT07 code coupled with the mathematical anthropomorphic phantoms CHILD-HIT and ADAM-HIT. The simulated irradiations were performed with primary ¹H, ⁴He, ⁷Li, ¹²C and ¹⁶O ion beams in the energy range 100–400 MeV/u which were directly impinging on the phantoms, i.e. approximating scanned beams, and with a simplified beamline for ¹²C irradiation. The evaluated absorbed doses to the out-of-field organs were in the range 10⁻⁶ to 10⁻¹ mGy per target Gy and with standard deviations 0.5–20%. While the contribution to the organ absorbed doses from secondary neutrons dominated in the ion beams of low atomic number Z, the produced charged fragments and their subsequent charged secondaries of higher generations became increasingly important for the secondary dose delivery as Z of the primary ions increased. As compared to the simulated scanned ¹²C ion beam, the implementation of a simplified beamline for prostate irradiation with ¹²C ions resulted in an increase of 2–50 times in the organ absorbed doses depending on the distance from the target volume. Comparison of secondary organ absorbed doses delivered by ¹H and ¹²C beams showed smaller differences when the RBE for local tumor control of the ions was considered and normalization to the RBE-weighted dose to the target was performed.

General scientific summary. During light ion therapy, the production of nuclear fragments results in a complex secondary radiation field which the organs and normal tissues of the patient are exposed to. In the present work, the absorbed doses to out-of-field organs and the energy distribution of secondary particle fluences in anthropomorphic phantoms have been simulated by the Monte Carlo code SHIELD-HIT07 for brain tumor and prostate irradiation with approximated scanned beams of ¹H, ⁴He, ⁷Li, ¹²C and 16O ions in the energy range 100–400 MeV/u, as well as with a simplified beam line for ¹²C irradiation. The evaluated organ absorbed doses were in the range 10⁻⁶ to 10⁻¹ mGy per target Gy. The absorbed dose contribution from secondary neutrons dominated in the ion beams of low atomic number Z, while the produced charged fragments and their subsequent charged secondaries became increasingly important for the secondary dose delivery as Z of the primary ion increased.

In light ion therapy, the knowledge of the spectra of both primary and secondary particles in the target volume is needed in order to accurately describe the treatment. The transport of ions in matter is complex and comprises both atomic and nuclear processes involving primary and secondary ions produced in the cascade of events. One of the critical issues in the simulation of ion transport is the modeling of inelastic nuclear reaction processes, in which projectile nuclei interact with target nuclei and give rise to nuclear fragments. In the Monte Carlo code SHIELD-HIT, inelastic nuclear reactions are described by the Many Stage Dynamical Model (MSDM), which includes models for the different stages of the interaction process. In this work, the capability of SHIELD-HIT to simulate the nuclear fragmentation of carbon ions in tissue-like materials was studied. The value of the parameter., which determines the so-called freeze-out volume in the Fermi break-up stage of the nuclear interaction process, was adjusted in order to achieve better agreement with experimental data. In this paper, results are shown both with the default value k = 1 and the modified value k = 10 which resulted in the best overall agreement. Comparisons with published experimental data were made in terms of total and partial charge-changing cross-sections generated by the MSDM, as well as integral and differential fragment yields simulated by SHIELD-HIT in intermediate and thick water targets irradiated with a beam of 400 MeV u(-1) C-12 ions. Better agreement with the experimental data was in general obtained with the modified parameter value (k = 10), both on the level of partial charge-changing cross-sections and fragment yields.

The dose-mean lineal energy, (y) over bar (D), has been calculated in water for irradiation with (12)C ions with initial energies 290 MeV/u. The y, was evaluated from the energy distributions of carbon and secondary boron ions, and from their energy-dependent pp-values. The energy distributions were obtained from simulations with the Monte Carlo code SHIELD-HIT07 and the energy-dependent (y) over bar (D)-values were obtained from ion-track simulations with PITS99 coupled with the electron transport code KURBUC. The ratio of the (y) over bar (D)-value determined in the vicinity of the Bragg peak to that calculated in a reference (60)Co gamma beam was compared with the corresponding ratio of alpha-values from the linear-quadratic model used in fractionated radiotherapy, showing a good correlation for an object size of around 10 nm.