Microdosimetry is a recommended method for characterizing radiation quality in situations when the biological effectiveness under test is not well known. In such situations, the radiation beams are described by their lineal energy probability distributions. Results from radiobiological investigations in the beams are then used to establish response functions that relate the lineal energy to the relative biological effectiveness (RBE). In this paper we present the influence of the size of the simulated volume on the relation to the clinical RBE values (or weighting factors). A single event probability distribution of the lineal energy is approximated by its dose average lineal energy (y[overline](D)) which can be measured or calculated for volumes from a few micrometres down to a few nanometres. The clinical RBE values were approximated as the ratio of the α-values derived from the LQ-relation. Model calculations are presented and discussed for the SOBP of a (12)C ion (290 MeV u(-1)) and the reference (60)Co γ therapy beam. Results were compared with those for a conventional x-ray therapy beam, a 290 MeV proton beam and a neutron therapy beam. It is concluded that for a simulated volume of about 10 nm, the α-ratio increases approximately linearly with the y[overline](D)-ratio for all the investigated beams. The correlation between y and α provides the evidence to characterize a radiation therapy beam by the lineal energy when, for instance, weighting factors are to be estimated.

The present investigation provides an insight into differences in energy depositions in subcellular-size volumes when irradiated by proton and carbon ion beams. The results are useful for characterizing ion beams of practical importance for biophysical modeling of radiation-induced DNA damage response and repair in the depth profiles of protons and carbon ions used in radiotherapy.

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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 (1)H, (4)He, (7)Li, (12)C and (16)O 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 (12)C 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 (12)C ion beam, the implementation of a simplified beamline for prostate irradiation with (12)C 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 (1)H and (12)C 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.

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 κ, 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 κ = 1 and the modified value κ = 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) (12)C ions. Better agreement with the experimental data was in general obtained with the modified parameter value (κ = 10), both on the level of partial charge-changing cross-sections and fragment yields.

The IAEA standard thermoluminescent dosimeter (TLD) holder has been developed for the IAEA/WHO TLD postal dose program for audits of high-energy photon beams, and it is also employed by the ESTRO-QUALity assurance network (EQUAL) and several national TLD audit networks. Factors correcting for the influence of the holder on the TL signal under reference conditions have been calculated in the present work from Monte Carlo simulations with the PENELOPE code for (60)Co gamma-rays and 4, 6, 10, 15, 18 and 25 MV photon beams. The simulation results are around 0.2% smaller than measured factors reported in the literature, but well within the combined standard uncertainties. The present study supports the use of the experimentally obtained holder correction factors in the determination of the absorbed dose to water from the TL readings; the factors calculated by means of Monte Carlo simulations may be adopted for the cases where there are no measured data.

The Monte Carlo codes show good agreement with experimental results for off-axis dose distributions. The disagreements in the Bragg peak region for the central-axis dose distributions imply that further improvements especially in the nuclear interaction models are required to increase the accuracy of the codes.

In a recent paper, the authors reported that the dose mean lineal energy, [Formula: see text] in a volume of about 10-15 nm is approximately proportional to the α-parameter in the linear-quadratic relation used in fractionated radiotherapy in both low- and high-LET beams. This was concluded after analyses of reported radiation weighting factors, WisoE (clinical RBE values), and [Formula: see text] values in a large range of volumes. Usually, microdosimetry measurements in the nanometer range are difficult; therefore, model calculations become necessary. In this paper, the authors discuss the calculation method. A combination of condensed history Monte Carlo and track structure techniques for calculation of mean lineal energy values turned out to be quite useful. Briefly, the method consists in weighting the relative dose fractions of the primary and secondary charged particles with their respective energy-dependent dose mean lineal energies. The latter were obtained using a large database of Monte Carlo track structure calculations.