Monte Carlo tools to supplement experimental microdosimetric spectra

TOPAS simulations of the response of a mini-TEPC: benchmark with experimental data

Objective. Microdosimetry offers a fast tool for radiation quality (RQ) verification to be implemented in treatment planning systems in proton therapy based on variable LET or RBE to move forward from the use of a fixed RBE of 1.1. It is known that the RBE of protons can increase up to 50% higher than that value in the last few millimetres of their range. Microdosimetry can be performed both experimentally and by means of Monte Carlo (MC) simulations. This paper has the aim of comparing the two approaches.Approach. Experimental measurements have been performed using a miniaturized Tissue equivalent proportional counter developed at the Legnaro National Laboratories of the Italian National Institute for Nuclear Physics with the aim of being used as RQ monitors for high intensity beams. MC simulations have been performed using the microdosimetric extension of TOPAS which provides optimized parameters and scorers for this application.Main results. Simulations were compared with experimental microdosimetric spectra in terms of shape of the spectra and their average values. Moreover, the latter have been investigated as possible estimators of LET obtained with the same MC code. The shape of the spectra is in general consistent with the experimental distributions and the average values of the distributions in both cases can predict the RQ increase with depth.Significance. This study aims at the comparison of microdosimetric spectra obtained from both experimental measurements and the microdosimetric extension of TOPAS in the same radiation field.

Patient-specific microdosimetry: a proof of concept

Microscopic energy deposition distributions from ionizing radiation are used to predict the biological effects of an irradiation and vary depending on biological target size. Ionizing radiation is thought to kill cells or inhibit cell cycling mainly by damaging DNA in the cell nucleus. The size of cells and nuclei depends on tissue type, cell cycle, and malignancy, all of which vary between patients. The aim of this study was to develop methods to perform patient-specific microdosimetry, that being, determining microdosimetric quantities in volumes that correspond to the sizes of cells and nuclei observed in a patient's tissue. A histopathological sample extracted from a stage I lung adenocarcinoma patient was analyzed. A pouring simulation was used to generate a three-dimensional tissue model from cell and nucleus size information determined from the histopathological sample. Microdosimetric distributions including f(y) and d(y) were determined for Co-60,Ir-192,Yb-169 and I-125 in a patient-specific model containing a distribution of cell and nucleus sizes. Fixed radius models and a summation method (where f(y) from many fixed radii models are summed) were compared to the full patient-specific model to evaluate their suitability for fast determination of patient-specific microdosimetric parameters. Fixed radius models do not provide a close approximation of the full patient-specific model y ̅_f or y ̅_d for the lower energy sources investigated, Yb-169 and I-125. The higher energy sources investigated, Co-60 and Ir-192 are less sensitive to target size variation than Yb-169 and I-125. A summation method yields the most accurate approximation of the full model d(y) for all radioisotopes investigated. A summation method allows for the computation of patient-specific microdosimetric distributions with the computing power of a personal computer. With appropriate biological inputs the microdosimetric distributions computed using these methods can yield a patient-specific relative biological effectiveness as part of a multiscale treatment planning approach.

Uncertainty in tissue equivalent proportional counter assessments of microdosimetry and RBE estimates in carbon radiotherapy

Microdosimetry is an important tool for assessing energy deposition distributions from ionizing radiation at cellular and cellular nucleus scales. It has served as an input parameter for multiple common mathematical models, including evaluation of relative biological effectiveness (RBE) of carbon ion therapy. The most common detector used for microdosimetry is the tissue-equivalent proportional counter (TEPC). Although it is widely applied, TEPC has various inherent uncertainties. Therefore, this work quantified the magnitude of TEPC measurement uncertainties and their impact on RBE estimates for therapeutic carbon beams. Microdosimetric spectra and frequency-, dose-, and saturation-corrected dose-mean lineal energy (****) were calculated using the Monte Carlo toolkit Geant4 for five monoenergetic and three spread-out Bragg peak carbon beams in water at every millimeter along the central beam axis. We simulated the following influences on these spectra from eight sources of uncertainty: wall effects, pulse pile-up, electronics, gas pressure, W-value, gain instability, low energy cut-off, and counting statistics. Statistic uncertainty was quantified as the standard deviation of perturbed values for each source. Bias was quantified as the difference between default lineal energy values and the mean of perturbed values for each systematic source. Uncertainties were propagated to RBE using the modified microdosimetric kinetic model (MKM). Variance introduced by statistic sources iny¯Fandy¯Daveraged 3.8% and 3.4%, respectively, and 1.5% iny*across beam depths and energies. Bias averaged 6.2% and 7.3% iny¯Fandy¯D,and 4.8% iny*.These uncertainties corresponded to 1.2 ± 0.9% on average in RBE_MKM. The largest contributors to variance and bias were pulse pile-up and wall effects. This study established an error budget for microdosimetric carbon measurements by quantifying uncertainty inherent to TEPC measurements. It is necessary to understand how robust the measurement of RBE model input parameters are against this uncertainty in order to verify clinical model implementation.

MONTE CARLO-BASED INVESTIGATION OF MICRODOSIMETRIC DISTRIBUTION OF HIGH ENERGY BRACHYTHERAPY SOURCES

FLUKA-based Monte Carlo calculations were carried out to study microdosimetric distributions in air and in water for encapsulated high energy brachytherapy sources (60Co, 137Cs, 192Ir and 169Yb) by simulating a Tissue Equivalent Proportional Counter (Model LET1/2) having sensitive diameter of 1. 27 cm for a site size of 1 μm. The study also included microdosimetric distributions of bare sources. When the sources are in air, for a given source, the source geometry does not affect the y¯F and y¯D values significantly. When the encapsulated 192Ir, 137Cs and 60Co sources are in water, y¯F and y¯D values increase with distance in water which is due to degradation in the energy of photons. Using the calculated values of y¯D, relative biological effectiveness (RBE) was obtained for the investigated sources. When 60Co, 137Cs and 192Ir sources are in water, RBE increases from 1.03 ± 0.01 to 1.17 ± 0.01, 1.24 ± 0.01 to 1.46 ± 0.02 and 1.50 ± 0.01 to 1.75 ± 0.03, respectively, when the distance was increased from 3-15 cm, whereas for 169Yb, RBE is about 2, independent of distance in water.

Characterization of a new tissue equivalent proportional counter for dosimetry of neutron and photon fields: comparison of measurements and Monte Carlo simulations

The tissue equivalent proportional counter (TEPC) is widely recognized as an important dosimetric technique particularly for complex radiation fields. The Korea Astronomy and Space Science Institute (KASI) has recently developed a new spherical TEPC to monitor the space radiation environment in the low earth orbit. The purpose of this study is to examine the performance of the TEPC against standard photon (¹³⁷Cs) and neutron (²⁵²Cf) sources through ground-based measurements and Monte Carlo simulations prior to its actual implementation. Lineal energy distributions, microdosimetric spectra and dosimetric quantities for a 2 µm simulated site in pure propane gas were determined for both sources. Both the measured and calculated ¹³⁷Cs spectra were shown to occur below 11 keV µm^-1 that is the typical range covered by photon sources. Complete coincidence of their electron edge regions was also observed. Meanwhile, the proton edge from the measured ²⁵²Cf spectra was found to be in good agreement with those from the simulated ones and the literature. The gamma, recoil proton and heavy ions peaks expected for neutron sources were well defined, albeit deviations in the gamma region. The absorbed dose and dose equivalent for both irradiation conditions were also successfully obtained. The dose equivalent for ²⁵²Cf was found to be ten times the absorbed dose whereas it remained the same for ¹³⁷Cs. The discrepancies observed in the low lineal energy region for both irradiation conditions were caused by intrinsic limitations on the experimental set-up and simulation configurations. This mainly contributed to a difference in the measured and calculated dose mean lineal energies of about 4.1% and 8.7% for the photon and neutron cases, respectively. Nevertheless, fair consistency with published data suggested that our TEPC could adequately reproduce the expected microdosimetric distributions for complex radiation fields.

MICRODOSIMETRIC SIMULATIONS OF CARBON IONS USING THE MONTE CARLO CODE FLUKA

Therapeutic carbon ion beams produce a complex and variable radiation field that changes along the penetration depth due to the high density of energy loss along the particle track together with the secondary particles produced by nuclear fragmentation reactions. An accurate physical characterisation of such complex mixed-radiation fields can be performed by measuring microdosimetric spectra with mini tissue-equivalent proportional counters (mini-TEPCs), which are one of the most accurate devices used in experimental microdosimetry. Numerical calculations with Monte Carlo codes such as FLUKA can be used to supplement experimental microdosimetric measurements performed with TEPCs, but the nuclear cross sections and fragmentation models need to be benchmarked with experimental data for different energies and scenarios. The aim of this work is to compare experimental carbon microdosimetric data measured with the mini TEPC with calculated microdosimetry spectra obtained with FLUKA for 12C ions of 189.5 MeV/u in the Bragg peak region.

Integrated Modelling of Cell Responses after Irradiation for DNA-Targeted Effects and Non-Targeted Effects

Intercellular communication after ionizing radiation exposure, so-called non-targeted effects (NTEs), reduces cell survival. Here we describe an integrated cell-killing model considering NTEs and DNA damage along radiation particle tracks, known as DNA-targeted effects (TEs) based on repair kinetics of DNA damage. The proposed model was applied to a series of experimental data, i.e., signal concentration, DNA damage kinetics, cell survival curve and medium transfer bystander effects (MTBEs). To reproduce the experimental data, the model considers the following assumptions: (i) the linear-quadratic (LQ) function as absorbed dose to express the hit probability to emit cell-killing signals, (ii) the potentially repair of DNA lesions induced by NTEs, and (iii) lower efficiency of repair for the damage in NTEs than that in TEs. By comparing the model results with experimental data, we found that signal-induced DNA damage and lower repair efficiency in non-hit cells are responsible for NTE-related repair kinetics of DNA damage, cell survival curve with low-dose hyper-radiosensitivity (HRS) and MTBEs. From the standpoint of modelling, the integrated cell-killing model with the LQ relation and a different repair function for NTEs provide a reasonable signal-emission probability and a new estimation of low-dose HRS linked to DNA repair efficiency.

FLUKA-BASED MONTE CARLO INVESTIGATION OF MICRODOSIMETRIC DISTRIBUTIONS OF TELECOBALT BEAM

FLUKA-based Monte Carlo calculations of microdosimetric distributions in water phantom involving a walled spherical Tissue-Equivalent Proportional Counter filled with tissue-equivalent propane gas have been studied for an indigenously developed telecobalt machine. The simulated site size considered in the study was 2 μm. In the Monte Carlo calculations, field size was varied from 10 cm × 10 cm to 35 cm × 35 cm and the depth was varied as 5-20 cm. The study also includes calculation of microdosimetric distributions with a 30° wedge filter. The efficiency of the calculations was improved up to a factor of 26 by choosing appropriate cut off values for production and transport of electrons. The calculated microdosimetric distributions of telecobalt machine is distinctly different from that of a bare 60Co source which is attributed to the influence of scattered photons from the machine head and the water phantom.

COMPARISON STUDY OF VARIOUS PLASTICS AS THE WALL MATERIAL OF THGEM-BASED MICRODOSEMETERS FOR FAST NEUTRON MEASUREMENTS

To find appropriate substitutions for the expensive plastics of A-150 and rexolite used in the construction of thick gas electron multiplier (THGEM)-based tissue-equivalent proportional counters, in the present work, the responses of a THGEM-based microdosimetric detector made of A-150 and rexolite and three others composed of plexiglas (PMMA), polyethylene and polystyrene plastics as the wall materials have been compared. Lineal energy distribution, frequency-averaged lineal energy, dose-averaged lineal energy, mean quality factor and dose-equivalent for 0.1, 1 and 10 MeV neutrons and also for 241Am-Be neutrons are calculated using Geant4 simulation toolkit. Frequency-averaged lineal energy, dose-averaged lineal energy, mean quality factor and dose-equivalent values for all plastics are found similar. In addition, the response of an indigenously constructed microdosemeter with PMMA walls is also measured for 241Am-Be neutrons. The experimental results are in good agreement with the simulation predictions. Conclusively, it was found that the three considered plastics can be used as good candidates instead of A-150 and rexolite plastics in fast neutron microdosimetry.

Reply to the comment on 'Monte Carlo calculated microdosimetric spread for cell nucleus-sized targets exposed to brachytherapy 125I and 192Ir sources and 60Co cell irradiation'

A discrepancy between the Monte Carlo derived relative standard deviation [Formula: see text] (microdosimetric spread) and experimental data was reported by Villegas et al (2013 Phys. Med. Biol. 58 6149-62) suggesting wall effects as a plausible explanation. The comment by Lindborg et al (2015 Phys. Med. Biol. 60 8621-4) concludes that this is not a likely explanation. A thorough investigation of the Monte Carlo (MC) transport code used for track simulation revealed a critical bug. The corrected MC version yielded [Formula: see text] values that are now within experimental uncertainty. Other microdosimetric findings are hereby communicated.