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Objective. This work investigates the use of passive luminescence detectors to determine different types of averaged linear energy transfer (LET-) for the energies relevant to proton therapy. The experimental results are compared to reference values obtained from Monte Carlo simulations. Approach. Optically stimulated luminescence detectors (OSLDs), fluorescent nuclear track detectors (FNTDs), and two different groups of thermoluminescence detectors (TLDs) were irradiated at four different radiation qualities. For each irradiation, the fluence- (LET-f) and dose-averaged LET (LET-d) were determined. For both quantities, two sub-types of averages were calculated, either considering the contributions from primary and secondary protons or from all protons and heavier, charged particles. Both simulated and experimental data were used in combination with a phenomenological model to estimate the relative biological effectiveness (RBE). Main results. All types ofLET-could be assessed with the luminescence detectors. The experimental determination ofLET-fis in agreement with reference data obtained from simulations across all measurement techniques and types of averaging. On the other hand,LET-dcan present challenges as a radiation quality metric to describe the detector response in mixed particle fields. However, excluding secondaries heavier than protons from theLET-dcalculation, as their contribution to the luminescence is suppressed by ionization quenching, leads to equal accuracy betweenLET-fandLET-d. Assessment of RBE through the experimentally determinedLET-dvalues agrees with independently acquired reference values, indicating that the investigated detectors can determineLET-with sufficient accuracy for proton therapy. Significance. OSLDs, TLDs, and FNTDs can be used to determineLET-and RBE in proton therapy. With the capability to determine dose through ionization quenching corrections derived fromLET-, OSLDs and TLDs can simultaneously ascertain dose,LET-, and RBE. This makes passive detectors appealing for measurements in phantoms to facilitate validation of clinical treatment plans or experiments related to proton therapy., (Creative Commons Attribution license.)

In radiation physics, Monte Carlo radiation transport simulations are powerful tools to evaluate the cellular responses after irradiation. When investigating such radiation-induced biological effects, it is essential to perform track structure simulations by explicitly considering each atomic interaction in liquid water at the sub-cellular and DNA scales. The Particle and Heavy-Ion Transport code System (PHITS) is a Monte Carlo code which enables to calculate track structure at DNA scale by employing the track-structure modes for electrons, protons and carbon ions. In this paper, we review the recent development status and future prospects of the track-structure modes in the PHITS code. To date, the physical features of these modes have been verified using the available experimental data and Monte Carlo simulation results reported in literature. These track-structure modes can be used for calculating microdosimetric distributions to estimate cell survival and for estimating initial DNA damage yields. The use of PHITS track-structure mode is expected not only to clarify the underlying mechanisms of radiation effects but also to predict curative effects in radiation therapy. The results of PHITS simulations coupled with biophysical models will contribute to the radiobiological studies by precisely predicting radiation-induced biological effects based on the Monte Carlo approach. [ABSTRACT FROM AUTHOR]

Changes in the relative biological effectiveness (RBE) of the radiation-induced cell killing of human salivary glands (HSG) were assessed along the Bragg peak of a 60 MeV clinical proton beam by means of coupling biophysical models with the results of Monte Carlo radiation transport simulations and experimental measurements with luminescent detectors. The fluence- and dose-mean unrestricted proton LET were determined along the Bragg peak using a recently developed methodology based on the combination of the response of 7LiF:Mg,Ti (MTS-7) and 7LiF:Mg,Cu,P (MCP-7) thermoluminescent detectors. The experimentally assessed LET values were compared with the results of radiation transport simulations using the Monte Carlo code PHITS, showing a good agreement. The cell survival probabilities and RBE were then calculated using the linear-quadratic model with the linear term derived using a phenomenological LET-based model (Carabe A et al 2012 Phys. Med. Biol. 57 1159) in combination with the experimentally-assessed or PHITS-simulated dose mean proton LET values. To the same aim, PHITS simulated microdosimetric spectra were used as input to the modified microdosimetric kinetic model (modified MKM, (Kase et al 2006 Radiat. Res. 166 629–38)). The RBE values calculated with the three aforementioned approaches were compared and found to be in very good agreement between each other, proving that by using dedicated pairs of thermoluminescent detectors it is possible to determine ionization density quantities of therapeutic proton beams which can be applied to predict the local value of the RBE. [ABSTRACT FROM AUTHOR]

Using a recently developed microdosimetric model based on the Monte Carlo code PHITS, the relative luminescence efficiency of LiF:Mg,Cu,P (MCP) thermoluminescent detectors to charged particles from 1H to 132Xe with energies from 3 to 1000 MeV/u has been assessed. The results have been compared with literature data showing very good agreement for particles from 12C to 132Xe in case of model calculations performed in a site size of 40 nm. For 1H and 4He ions, the model overestimates the experimental values by ~10%. An explanation of this deviation as a consequence of fading effects has been included. [ABSTRACT FROM AUTHOR]

The relative efficiency of the two most commonly used 7LiF:Mg,Ti (MTS-7) and 7LiF:Mg,Cu,P (MCP-7) thermoluminescent detectors was calculated using the Microdosimetric d(z) Model for negative muons, positive muons, negative pions, positive pions, negative kaons, positive kaons and neutral kaons with energies spanning from 2 keV to 1 GeV. The needed microdosimetric specific energy probability density distributions were obtained by performing Monte Carlo radiation transport simulations using the computer code PHITS in the optimal 40 nm site size. The obtained efficiency values were found to be strongly affected by the particle type, its energy and the detector, ranging from 0.25 to 1.08. Possible correlations with average microdosimetric quantities such as the frequency- and dose-mean specific energy were discussed in detail. Finally, the effect of including the nominal dopant concentrations in the simulations used for the assessment of the microdosimetric spectra was proven to play a marginal role in the computation of the relative efficiency values, in agreement with what was found in previous studies for ions, photons, electrons, positrons and mixed particle fields. • Specific energy density distributions for muons, pions and kaons were simulated using PHITS. • The obtained spectra were used as input to the Microdosimetric d(z) Model. • The efficiencies of 7LiF:Mg,Ti (MTS-7) and (MCP-7) detectors were theoretically calculated. • The efficiency values depend strongly on the detector, the particle type and its energy. [ABSTRACT FROM AUTHOR]

The MicroPlus Bridge V2 silicon microdosimeter was exposed to collimated protons in a clinical radiotherapy beam, a mixed photon–neutron radiation field from a sealed 252Cf source and γ -rays from a 137Cs source in order to investigate the accuracy and the uncertainty budget associated with the calibration of this detector by means of the proton-edge technique. At first, the energy values associated with the proton- and electron-edges were assessed for the detector under study by performing radiation transport simulations using the Monte Carlo code PHITS. After calibrating the detector in pulse amplitude using a pulse generator and in energy imparted using the PHITS-determined proton-edge, the accuracy of the calibration was tested by comparing the position of the electron-edge in the experimental microdosimetric spectra with the theoretical value obtained using PHITS. A study on the determination of which marker point (inflection point, maximum of the second derivative, intercept of the tangent through the inflection point) is the most accurate and least affected by the arbitrary choice of the fitting range is included in the article, proving that the detector can be successfully calibrated using the proton-edge technique with a combined uncertainty of 4%. [ABSTRACT FROM AUTHOR]

The proton depth dose profile measured by luminescent detectors differs from the one measured with reference dosimeters (i.e. ionization chambers) because of several effects including efficiency quenching and changes in the attenuation of the emitted light in case of partial detector irradiation. Using the Microdosimetric d(z) Model in combination with the Monte Carlo radiation transport code PHITS, a methodology was developed to tackle all these factors and calculate the response of luminescent when exposed at different positions along a proton Bragg peak. The results were compared against experimental data gathered with 7LiF:Mg,Ti (MTS-7) and 7LiF:Mg,Cu,P (MCP-7) thermoluminescent detectors, showing a very good agreement (average relative deviation ~ 3% for both detector types, smaller than the combined experimental uncertainty). • The Microdosimetric d(z) Model was coupled with nanoscale simulations using PHITS. • The response of luminescent detectors was predicted along a clinical proton Bragg peak. • A very good agreement with experimental data was found. • The Microdosimetric d(z) Model can be reliably used for calculations in mixed fields. [ABSTRACT FROM AUTHOR]

7LiF:Mg,Cu,P (MCP-7) thermoluminescent detectors were exposed at nine positions within a 60 MeV pristine proton Bragg peak to investigate the possibility of counteracting the occurrence of their low temperature anomalous behavior (strong increase in the relative intensity of the peak 3 after exposure to energetic charged particles, Parisi et al., 2018 a) by using the post-irradiation pre-readout thermal protocol of 120 °C for 30 min. Simulations with the Monte Carlo radiation transport code PHITS were performed in order to determine the absorbed dose, the LET and the specific energy density distributions within the detector volume as a function of its position along the Bragg peak. The experimentally determined efficiency values for the main peak signal agreed with the results of the Microdosimetric d(z) Model with an average deviation of 2.4%. The consistency of the results supports the hypothesis that the light signal of the main peak 4 partly arises from trapped charge carriers previously giving rise to the neighboring low-temperature peak 3 and that the non-standardized post-irradiation procedure is the reason for the spread in the experimentally determined proton efficiency data present in literature. Thus, for a correct dose assessment and to mitigate possible low temperature anomalies in the glow curve structure of LiF:Mg,Cu,P detectors, it is strongly advised to always employ the recommended preheat protocol of 30 min at 120 °C. • Using an optimal pre-heat protocol, LiF:Mg,Cu,P efficiency to protons was determined. • A very good agreement with theoretical efficiency values was found. • The low temperature anomalous behavior was successfully counteracted. • The phenomenon was explained as charge carrier migration from peak 3 to peak 4. [ABSTRACT FROM AUTHOR]

An intercomparison of microdosimetric and nanodosimetric quantities simulated Monte Carlo codes is in progress with the goal of assessing the uncertainty contribution to simulated results due to the uncertainties of the electron interaction cross-sections used in the codes. In the first stage of the intercomparison, significant discrepancies were found for nanodosimetric quantities as well as for microdosimetric simulations of a radiation source placed at the surface of a spherical water scoring volume. This paper reports insight gained from further analysis, including additional results for the microdosimetry case where the observed discrepancies in the simulated distributions could be traced back to the difference between track-structure and condensed-history approaches. Furthermore, detailed investigations into the sensitivity of nanodosimetric distributions to alterations in inelastic electron scattering cross-sections are presented which were conducted in the lead up to the definition of an approach to be used in the second stage of the intercomparison to come. The suitability of simulation results for assessing the sought uncertainty contributions from cross-sections is discussed and a proposed framework is described. • Intercomparison track structure codes in micro and nanodosimetry. • Nanodosimetric results spread related to Low energy electron cross-sections uncertainty. • Method for assessing uncertainty of simulated results in nanodosimetry (no experimental data available for comparison). [ABSTRACT FROM AUTHOR]