Mean Lineal Energy Research Articles

Status of LET assessment with active and passive detectors in ion beams

This review explores current experimental methods for determining the radiation quality in ion beams. In this context, radiation quality is commonly evaluated using the averaged linear energy transfer (LET), a metric employed to assess the response of both biological and physical systems. Dose and averaged LET can be experimentally determined with passive detectors through various techniques that have seen recent improvements. Another metric related to the LET is the mean lineal energy, which is measurable using microdosimetric detectors. This review focuses on the available possibilities for evaluating the radiation quality using three microdosimeters (mini-TEPC, Silicon Telescope, and SOI Microplus), three passive luminescence detectors (based on optical, thermo-, and radiophoto-luminescence), three track-based detectors (track-etched detector, Timepix, fluorescent nuclear track detector), and a chemical detector based on alanine. A comparison of detector properties is provided along with an overview of the underlying mechanisms enabling LET assessment or measurements of the mean lineal energy with each detector type. Finally, this review summarizes the current possibilities of LET determination with respect to the needs for quality assurance in particle therapy. Areas for future research and development are suggested.

Lithium inelastic cross-sections and their impact on micro and nano dosimetry of boron neutron capture

Objective. To present a new set of lithium-ion cross-sections for (i) ionization and excitation processes down to 700 eV, and (ii) charge-exchange processes down to 1 keV u−1. To evaluate the impact of the use of these cross-sections on micro a nano dosimetric quantities in the context of boron neutron capture (BNC) applications/techniques. Approach. The Classical Trajectory Monte Carlo method was used to calculate Li ion charge-exchange cross sections in the energy range of 1 keV u−1 to 10 MeV u−1. Partial Li ion charge states ionization and excitation cross-sections were calculated using a detailed charge screening factor. The cross-sections were implemented in Geant4-DNA v10.07 and simulations and verified using TOPAS-nBio by calculating stopping power and continuous slowing down approximation (CSDA) range against data from ICRU and SRIM. Further microdosimetric and nanodosimetric calculations were performed to quantify differences against other simulation approaches for low energy Li ions. These calculations were: lineal energy spectra (yf(y) and yd(y)), frequency mean lineal energy yF― , dose mean lineal energy yD― and ionization cluster size distribution analysis. Microdosimetric calculations were compared against a previous MC study that neglected charge-exchange and excitation processes. Nanodosimetric results were compared against pure ionization scaled cross-sections calculations. Main results. Calculated stopping power differences between ICRU and Geant4-DNA decreased from 33.78% to 6.9%. The CSDA range difference decreased from 621% to 34% when compared against SRIM calculations. Geant4-DNA/TOPAS calculated dose mean lineal energy differed by 128% from the previous Monte Carlo. Ionization cluster size frequency distributions for Li ions differed by 76%–344.11% for 21 keV and 2 MeV respectively. With a decrease in the N 1 within 9% at 10 keV and agreeing after the 100 keV. With the new set of cross-sections being able to better simulate low energy behaviors of Li ions. Significance. This work shows an increase in detail gained from the use of a more complete set of low energy cross-sections which include charge exchange processes. Significant differences to previous simulation results were found at the microdosimetric and nanodosimetric scales that suggest that Li ions cause less ionizations per path length traveled but with more energy deposits. Microdosimetry results suggest that the BNC’s contribution to cellular death may be mainly due to alpha particle production when boron-based drugs are distributed in the cellular membrane and beyond and by Li when it is at the cell cytoplasm regions.

Microdosimetric analysis of monoenergetic electrons and beta-emitting radionuclides based on mesh-type cell models and RBE prediction

AimsMicrodosimetric indicators are commonly used to describe the energy deposition characteristics of ionizing radiation at the subcellular and DNA levels to predict radiation-induced biological effects. Monoenergetic electrons and beta-emitting radionuclides (β nuclides) have a certain range in low-density media, and their microdosimetric indicators in the cell nucleus are of great significance for understanding the characteristics of different types of β nuclides and the evaluation of relative biological effectiveness (RBE). MethodsMicrodosimetry in the nucleus is greatly affected by cell morphology, so in this paper, based on laser confocal tomography images of BEAS-2B and L-O2 cell lines, two types of mesh-type cell models were constructed to restore the morphological features of real cells. The Geant4-DNA toolkit was used to simulate the lineal energy and dose distribution of monoenergetic electrons (0.1 keV-1 MeV) and 5 types of beta nuclides (Cs-137, Cs-134, C-14, I-131, and H-3) in the cell nucleus with different initial energies. The effects of factors such as initial energy, sensitive site (radius = 2 nm, 10 nm, 30 nm, 300 nm), and cell type on microdosimetric quantities in the nucleus and their predictions of nuclide RBE were analysed. ResultsThe results show that for the same type of nuclide, there are different RBE values between BEAS-2B and L-O2 cells, with Cs-134 having a difference of 17.75% and H-3 having a difference of 10.58%. In addition, the mean lineal energy in the nucleus fluctuates with changes in the initial energy and sampling site radius of monoenergetic electrons, especially for low and medium energies where the sampling site radius has a significant effect. Moreover, the proximity functions of monoenergetic electrons with an initial energy below 1 keV are greatly affected by the cell shape, and those above 20 keV are greatly affected by the model volume. ConclusionsThe study confirms the necessity of using mesh cell models for microdosimetric analysis in the cell nucleus and predicting its biological effects by combining mesh models of real human cells to simulate microdosimetric indicators of monoenergetic electrons and beta-emitting nuclides. The estimation of the RBE of β nuclides achieved in this study has reference value for predicting radiobiological damage in different cell types.

Diamond based integrated detection system for dosimetric and microdosimetric characterization of radiotherapy ion beams.

Ion beam therapy allows for a substantial sparing of normal tissues and higher biological efficacy. Synthetic single crystal diamond is a very good material to produce high-spatial-resolution and highly radiation hard detectors for both dosimetry and microdosimetry in ion beam therapy. The aim of this work is the design, fabrication and test of an integrated waterproof detector based on synthetic single crystal diamond able to simultaneously perform dosimetric and microdosimetric characterization of clinical ion beams. The active elements of the integrated diamond device, that is, dosimeter and microdosimeter, were both realized in a Schottky diode configuration featured by different area, thickness, and shape by means of photolithography technologies for the selective growth of intrinsic and boron-doped CVD diamond. The cross-section of the sensitive volume of the dosimetric element is 4mm2 and 1μm-thick, while the microdosimetric one has an active cross-sectional area of 100×100μm2 and a thickness of about 6.2μm. The dosimetric and microdosimetric performance of the developed device was assessed at different depths in a water phantom at the MedAustron ion beam therapy facility using a monoenergetic uniformly scanned carbon ion beam of 284.7MeV/u and proton beam of 148.7MeV. The particle flux in the region of the microdosimeter was 6·107 cm2 /s for both irradiation fields. At each depth, dose and dose distributions in lineal energy were measured simultaneously and the dose mean lineal energy values were then calculated. Monte Carlo simulations were also carried out by using the GATE-Geant4 code to evaluate the relative dose, dose averaged linear energy transfer (LETd ), and microdosimetric spectra at various depths in water for the radiation fields used, by considering the contribution from the secondary particles generated in the ion interaction processes as well. Dosimetric and microdosimetric quantities were measured by the developed prototype with relatively low noise (∼2keV/μm). A good agreement between the measured and simulated dose profiles was found, with discrepancies in the peak to plateau ratio of about 3% and 4% for proton and carbon ion beams respectively, showing a negligible LET dependence of the dosimetric element of the device. The microdosimetric spectra were validated with Monte Carlo simulations and a good agreement between the spectra shapes and positions was found. Dose mean lineal energy values were found to be in close agreement with those reported in the literature for clinical ion beams, showing a sharp increase along the Bragg curve, being also consistent with the calculated LETd for all depths within the experimental error of 10%. The experimental indicate that the proposed device can allow enhanced dosimetry in particle therapy centers, where the absorbed dose measurement is implemented by the microdosimetric characterization of the radiation field, thus providing complementary results. In addition, the proposed device allows for the reduction of the experimental uncertainties associated with detector positioning and could facilitate the partial overcoming of some drawbacks related to the low sensitivity of diamond microdosimeters to low LET radiation.

3D printed 2D range modulators preserve radiation quality on a microdosimetric scale in proton and carbon ion beams

IntroductionParticle therapy using pencil beam scanning (PBS) faces large uncertain- ties related to ranges and target motion. One possibility to improve existing mitigation strategies is a 2D range modulator (2DRM). A 2DRM offers faster irradiation times by reducing the number of layers and spots needed to create a spread-out Bragg peak. We have investigated the impact of 2DRM on microdosimetric spectra measured in proton and carbon ion beams. Materials and MethodsTwo 2DRMs were designed and 3D printed, one for.124.7 MeV protons and one for 238.6 MeV/u carbon ions. Their dosimetric validation was performed using Roos and PinPoint ionization chamber and EBT3 films. Monte Carlo simulations were done using GATE. A silicon-based solid-state microdosimeter was used to collect pulse-height spectra along three depths for two irradiation modalities, PBS and a single central beam. ResultsFor both particle types, the original pin design had to be optimized via GATE simulations. The difference between the R80 of the simulated and measured depth dose curve was 0.1 mm. The microdosimetric spectra collected with the two irradiation modalities overlap well. Their mean lineal energy values differ over all positions by 5.2 % for the proton 2DRM and 2.1 % for the carbon ion 2DRM. ConclusionRadiation quality in terms of lineal energy was independent of the irradiation method. This supports the current approach in reference dosimetry, where the residual range is chosen as a beam quality index to select stopping power ratios.

Microdosimetry with a mini-TEPC in the spread-out Bragg peak of 148 MeV protons

A new miniaturized Tissue Equivalent Proportional Counter (mini‒TEPC) with cylindrical sensitive volume of 1.0 mm in diameter and height, and with external diameter of 1.53 cm, has been developed to work without gas flow. With this mini counter we have measured the radiation quality of the 148 MeV energy-modulated proton beam at the radiobiological research line of the Trento Proton Therapy Centre. The irradiation field had a diameter of about 6 cm, a modulation width of 2.5 cm and a range of about 13.5 cm. At four specific depths, the response function of the new microdosimeter has also been compared to that of a commercial spherical microdosimeter, the LET-1/2” Spherical TEPC produced by Far West Technology. Afterwards, measurements were performed with the mini‒TEPC at nine precise positions across the Spread-Out Bragg Peak (SOBP).From the microdosimetric spectra the frequency and the dose‒mean lineal energy values, y‾F and y‾D, were derived for both detectors. Afterwards, microdosimetric assessment of Relative Biological Effectiveness (RBEμ) has been performed by weighting the dose distribution of the lineal energy, d(y), with the Loncol’s biological weighting function r(y).RBEμ has been used to calculate the RBEμD at different dose levels D. In the distal fall-off region the RBEμD(d) increases significantly, up to a factor 3. Finally, the RBEμD-weighted dose was calculated and compared to that obtained with a fixed RBE = 1.1. The use of a variable RBE to weight the physical dose leads to a biological range extension evaluated at R80 of about 2.0 mm, with respect to the R80 obtained with a fixed RBE = 1.1.The use of a mini‒TEPC in proton therapy has been so far limited to 60 MeV proton beams, used for the treatment of ocular melanoma. This is the first time that a mini-TEPC was used with an higher energy proton beam of 148 MeV, and the spectra measured with the mini-TEPC were compared to those measured with the commercial FWT LET1/2 counter for the first time.These new results corroborate the usability of mini-TEPCs in proton therapy.

Experimental and Monte Carlo based study of different microdosimetric quantities at mixed radiation environments of nuclear reactor, reprocessing facility and D-D accelerator

Personnel working in the environment of nuclear reactors, fuel reprocessing facilities, high energy accelerators get exposed to a mixed radiation field of electrons, photons, and neutrons with a wide range of energy and fluence. Performance evaluations of the passive personnel dosimeters in these mixed radiation environments require knowledge of the external dose contributed by different types of radiations having variation in their energy, etc. Tissue Equivalent Proportional Counters (TEPC), sensitive to a wide range of particles and energy (Linear Energy Transfer (LET) ∼ 0–1500 keV μm−1), is one of the standard reference detectors which can be used for the characterization of mixed radiation fields. In the present work, HAWK TEPC was used to evaluate different microdosimetric parameters such as dose mean lineal energy (ӯD), average quality factor (Ǭ), the contribution of gamma and neutron dose equivalents for the characterizations of the radiation fields inside in a nuclear research reactor hall, fuel reprocessing facility, D-D neutron beam facility, radionuclide source based neutron calibration facilities, etc. Measured results were compared with Monte Carlo simulated values using radiation transport code PHITS. TEPC measurement shows that the radiation work field around nuclear reactor facility is dominated by gamma rays with approximate ӯD and Ǭ of ∼42 keV μm−1 and ∼2.3, respectively. Whereas ӯD and Ǭ values for neutron dominated locations in fuel reprocessing facility are 82 keV μm−1 and 8.2, respectively. Ǭ values near thermal neutron beam line in research reactor are close to 1 due to (n, γ) reaction. For 2.54 MeV mono-energetic D-D neutron beam, measured and calculated dose mean lineal energies (ӯD) are 52.5 keV μm−1 and 59.6 keV μm−1, respectively, whereas corresponding Ǭ values are 11.23 and 13.01. Obtained values can be utilized to correlate the neutron and gamma dose equivalents measured by passive detectors used for personnel monitoring program to enhance confidence in the dosimetry.

In-field and out-of-field microdosimetric characterisation of a 62 MeV proton beam at CATANA.

A 5 and 10 μm thin silicon on insulator (SOI) 3D mushroom microdosimeter was used to characterize both the in-field and out-of-field of a 62 MeV proton beam. The SOI mushroom microdosimeter consisted of an array of cylindrical sensitive volumes (SVs), developed by the Centre for Medical Radiation Physics, University of Wollongong, was irradiated with 62 MeV protons at the CATANA (Centro di AdroTerapia Applicazioni Nucleari Avanzate) facility in Catania, Italy, a facility dedicated to the radiation treatment of ocular melanomas. Dose mean lineal energy, ( ), values were obtained at various depths in PMMA along a pristine and spread out Bragg peak (SOBP). The measured microdosimetric spectra at each position were then used as inputs into the modified Microdosimetric Kinetic Model (MKM) to derive the RBE for absorbed dose in a middle of the SOBP 2Gy (RBED ). Microdosimetric spectra were obtained with both the 5 and 10 μm 3D SOI microdosimeters, with a focus on the distal part of the BP. The in-field and out-of-field measurement configurations along the Bragg curve were modeled in Geant4 for comparison with experimental results. Lateral out-of-field measurements were performed to study secondary particles' contribution to normal tissue's dose, up to 12 mm from the edge of the beam field, and quality factor and dose equivalent results were obtained. Comparison between experimental and simulation results showed good agreement between one another for both the pristine and SOBP beams in terms of and RBED. Though a small discrepancy between experiment and simulation was seen at the entrance of the Bragg curve, where experimental results were slightly lower than Geant4. The dose equivalent value measured 12 mm from the edge of the target volume was 1.27 ± 0.15 mSv/Gy with a value of 2.52 ± 0.30, both of which agree within uncertainty with Geant4 simulation. These results demonstrate that SOI microdosimeters are an effective tool to predict RBED in-field as well as dose equivalent monitoring out-of-field to provide insight to probability of second cancer generation.

Calculation of microdosimetric spectra for protons using Geant4-DNA and a μ-randomness sampling algorithm for the nanometric structures

Purpose Through introducing stochastic quantities that can be connected to the dimensions of the microscopic structures exposed to radiations, microdosimetry is concerned with the substantive specifications of radiation quality that could help gain insight into radiation effects. Utilizing the μ-randomness method and Geant4-DNA code, we calculated microdosimetry quantities for nanometric structures in a spherical body of water irradiated with protons. To gain more insight into the effects of radiation on microscopic structures and validate the code parameters, we made a comparison between our results obtained within Geant4-DNA and results from other simulations. Materials and methods We calculated microdosimetric quantities through irradiating a spherical body of water of 6 μm diameter with 0.5–100 MeV protons. Microdosimetric quantities were derived for cylinders with diameter × height values of 23 × 23, 50 × 100, and 300 × 300 Å × Å, which would resemble the typical sizes of sub-cellular organisms such as the DNA, nucleosome, and chromatin fiber. We exploited the concept of μ-randomness to introduce convex bodies of random positions and directions for calculating microdosimetric quantities. We used the Geant4-DNA Monte Carlo simulation toolkit for transporting protons and secondary particles and calculating the frequency- and dose-mean lineal and specific energies in cylindrical volumes. Specifically, for same-sized cylindrical volumes, microdosimetric parameters obtained by Nikjoo et al. using the KURBUC code were used for evaluation. Results For the energy range investigated, the frequency-mean lineal energy, dose-mean lineal energy, frequency-mean specific energy, and dose-mean specific energy vary within [2.34,47.06] (keV/μm), [10.40,68.55] (keV/μm), [0.04,39.38] × 106 cGy, and [0.16,90.29] × 106 cGy, respectively. Regardless of the proton energy, our specific-energy results showed higher sensitivity to volume change, for smaller cylinder volumes rather than larger ones. Regardless of both proton energy and volume of the cylinder under study, we observed a generally better agreement between our frequency-mean, than dose-mean, specific energy results and the KURBUC results. Conclusion Using Geant4-DNA to account for the stochastic nature of energy depositions due to physical interactions between radiation and matter, we calculated microdosimetry parameters concerning proton irradiation. By employing microdosimetry concepts in conjunction with simulation results of our previous work on radiation effects on the DNA, we pinpointed and quantified correlations between microdosimetry parameters and DNA damage. As such, for a volume with comparable mass and mean chord length to the DNA, we could observe the clear correspondence of the mean lineal and specific energy results with the double-strand-break yields of protons in Gy−1.Gbp−1.

Microdosimetric measurements of a monoenergetic and modulated Bragg Peaks of 62MeV therapeutic proton beam with a synthetic single crystal diamond microdosimeter.

The purpose of this study was to investigate for the first time the performance of a synthetic single crystal diamond detector for the microdosimetric characterization of clinical 62MeV ocular therapy proton beams. A novel diamond microdosimeter with a well-defined sensitive volume was fabricated and tested with a monoenergetic and spread-out Bragg peak (SOBP) of the CATANA therapeutic proton beam in Catania, Italy. The whole sensitive volume of the detector has an active planar-sectional area of 100µm×100µm and a thickness of approximately 6.3 um. Microdosimetric measurements were performed at several water equivalent depths, corresponding to positions of clinical relevance. From the measured spectra, microdosimetric quantities such as the frequency mean lineal energy ( ), dose mean lineal energy ( ) as well as microdosimetric relative biological effectiveness (RBEµ ) values were derived for each depth along both a pristine Bragg curve and SOBP. Finally, Geant4 Monte Carlo simulations were performed modeling the detector geometry and CATANA beamline in order to calculate the average linear energy transfer (LET) values in the diamond active layer and water. The microdosimetric spectra acquired by the diamond microdosimeter show different shapes as a function of the water equivalent depths. No spectral distortion, due to pile-up events and polarization effects, was observed. The experimental spectra have a very low detection threshold due to the electronic noise during the irradiation of about 1keV/μm. The and values were in agreement with expected trends, showing a sharp increase in mean lineal energy at the distal edge of the Bragg peak. In addition, a good agreement between the mean lineal energy values and the calculated average LET ones was also observed. Finally, the RBE values evaluated with the diamond microdosimeter were in excellent agreement with those obtained with a mini tissue equivalent proportional counter as well as with radiobiological measurements in the same proton beam field. The microdosimetric performance of the tested synthetic single crystal diamond microdosimeter clearly indicates its suitability for quality assurance in clinical proton therapy beam.

Validation of Geant4 for silicon microdosimetry in heavy ion therapy

Microdosimetry is a particularly powerful method to estimate the relative biological effectiveness (RBE) of any mixed radiation field. This is particularly convenient for therapeutic heavy ion therapy (HIT) beams, referring to ions larger than protons, where the RBE of the beam can vary significantly along the Bragg curve. Additionally, due to the sharp dose gradients at the end of the Bragg peak (BP), or spread out BP, to make accurate measurements and estimations of the biological properties of a beam a high spatial resolution is required, less than a millimetre. This requirement makes silicon microdosimetry particularly attractive due to the thicknesses of the sensitive volumes commonly being ∼10 m or less. Monte Carlo (MC) codes are widely used to study the complex mixed HIT radiation field as well as to model the response of novel microdosimeter detectors when irradiated with HIT beams. Therefore it is essential to validate MC codes against experimental measurements.This work compares measurements performed with a silicon microdosimeter in mono-energetic , and ion beams of therapeutic energies, against simulation results calculated with the Geant4 toolkit. Experimental and simulation results were compared in terms of microdosimetric spectra (dose lineal energy, ), the dose mean lineal energy, y D and the RBE10, as estimated by the microdosimetric kinetic model (MKM). Overall Geant4 showed reasonable agreement with experimental measurements. Before the distal edge of the BP, simulation and experiment agreed within ∼10% for y D and ∼2% for RBE10. Downstream of the BP less agreement was observed between simulation and experiment, particularly for the and beams. Simulation results downstream of the BP had lower values of y D and RBE10 compared to the experiment due to a higher contribution from lighter fragments compared to heavier fragments.

Microdosimetry with a sealed mini-TEPC and a silicon telescope at a clinical proton SOBP of CATANA

A sealed miniaturized Tissue Equivalent Proportional Counter (mini-TEPC) able to work in gas-steady modality was developed at the Legnaro National Laboratories of the Italian National Institute of Nuclear Physics (LNL – INFN, Legnaro, Italy).The aim of the present work is to compare the response of this mini-TEPC with that of a silicon microdosimeter based on a monolithic telescope.Pairwise measurements were performed at the 62 MeV proton clinical Spread Out Bragg Peak (SOBP) of CATANA at the Southern National Laboratories of INFN (LNS – INFN, Catania, Italy). The dose mean lineal energy values were derived from the spectra measured with the two detectors and compared with the total dose-averaged LET calculated by means of Geant4 Monte Carlo simulations. Finally, the possibility to apply the Microdosimetric Kinetic Model (MKM) to reproduce RBE variations with depth along the Spread Out Bragg Peak was investigated.

The impact of sensitive volume thickness for silicon on insulator microdosimeters in hadron therapy

Compact silicon on insulator (SOI) microdosimeters have been used to characterise the radiation field of many different hadron therapy beams. SOI devices are particularly attractive in hadron therapy fields due to their spatial resolution being well suited to the sharp dose gradients at the end of the primary beam’s range. Due to the small size of SOI’s sensitive volumes (SVs), which are usually ∼1–10 m thick, the fabrication of these devices can present challenges which are not as common for more conventional thickness silicon devices such as silicon spectroscopy detectors. Microdosimetry is the study of the energy deposition in micrometre sized volumes representing biological sites and is a powerful approach to estimate the biological effect of radiation on the micron-scale level, in a cell. However, cell sizes vary extensively translating in different energy deposition spectra. This work studies SV thicknesses between 1 and 100 m using Geant4 and examines the impact of SV dimensions on microdosimetric quantities. The quantities studied were the frequency mean lineal energy, , and the dose mean lineal energy, . Additionally the relative biological effectiveness (RBE), estimated by the microdosimetric kinetic model (MKM), is also investigated. To study the impact of the SV thickness, SOI microdosimeters were irradiated with proton, and ion beams with ranges of ∼160 mm, with the microdosimeter being set at various positions along the Bragg curve. It was found that was influenced the least in proton beams and increased for heavier ion beams. Conversely, was impacted by the SV thickness the most in proton beams and was the least. Similar to , protons were impacted the most by the SV thickness when estimating the RBE using the MKM. The cause of these differences was largely due to the different densities of the delta electron track structure for the case of and the energy transferred to the medium from the primary beam for .

Validation of linear energy transfer computed in a Monte Carlo dose engine of a commercial treatment planning system

The relative biological effectiveness (RBE) of protons is highly variable and difficult to quantify. However, RBE is related to the local ionization density, which can be related to the physical measurable dose weighted linear energy transfer (LETD). The aim of this study was to validate the LETD calculations for proton therapy beams implemented in a commercially available treatment planning system (TPS) using microdosimetry measurements and independent LETD calculations (Open-MCsquare (MCS)).The TPS (RayStation v6R) was used to generate treatment plans on the CIRS-731-HN anthropomorphic phantom for three anatomical sites (brain, nasopharynx, neck) for a spherical target (Ø = 5 cm) with uniform target dose to calculate the LETD distribution. Measurements were performed at the University Medical Center Groningen proton therapy center (Proteus Plus, IBA) using a µ+-probe utilizing silicon on insulator microdosimeters capable of detecting lineal energies as low as 0.15 keV µm−1 in tissue. Dose averaged mean lineal energy depth-profiles were measured for 70 and 130 MeV spots in water and for the three treatment plans in water and an anthropomorphic phantom. The measurements were compared to the LETD calculated in the TPS and MCS independent dose calculation engine. D · was compared to D · LETD in terms of a gamma-index with a distance-to-agreement criteria of 2 mm and increasing dose difference criteria to determine the criteria for which a 90% pass rate was accomplished.Measurements of D · were in good agreement with the D · LETD calculated in the TPS and MCS. The 90% passing rate threshold was reached at different D · LETD difference criteria for single spots (TPS: 1% MCS: 1%), treatment plans in water (TPS: 3% MCS: 6%) and treatment plans in an anthropomorphic phantom (TPS: 6% MCS: 1%).We conclude that D · LETD calculations accuracy in the RayStation TPS and open MCSquare are within 6%, and sufficient for clinical D · LETD evaluation and optimization. These findings remove an important obstacle in the road towards clinical implementation of D · LETD evaluation and optimization of proton therapy treatment plans.Novelty and significanceThe dose weighed linear energy transfer (LETD) distribution can be calculated for proton therapy treatment plans by Monte Carlo dose engines. The relative biological effectiveness (RBE) of protons is known to vary with the LETD distribution. Therefore, there exists a need for accurate calculation of clinical LETD distributions. Previous LETD validations have focused on general purpose Monte Carlo dose engines which are typically not used clinically. We present the first validation of mean lineal energy measurements of the LETD against calculations by the Monte Carlo dose engines of the Raystation treatment planning system and open MCSquare.

The influence of Geant4-DNA toolkit parameters on electron microdosimetric track structure.

The influence of different physical process factors on tracks of low-energy electrons in liquid water was analyzed and evaluated based on the Geant4-DNA toolkit of Geant4 version 10.4, and it provides theoretical support for obtaining the basic parameters of microdosimetry concerned with radiotherapy and radiation protection. According to the characteristics of different models, five physics constructors of Geant4-DNA toolkit were selected to simulate monoenergetic electrons in microscopic scale. Details of track structure of different Geant4-DNA physics constructors were compared, including total number of interaction processes, number and energy percentage of excitation and ionization; analyzing the impacts of mean lineal energy of several factors, including Geant4-DNA physics constructors, initial energy, radius of scoring spheres, interaction processes and cut-off energy. Firstly, ‘G4EmDNAPhysics’ (hereinafter referred to as ‘dna’) is well consistent with ‘G4EmDNAPhysics_option 2’ (hereinafter referred to as ‘option 2’), and ‘G4EmDNAPhysics_option 4’ (hereinafter referred to as ‘option 4’) is well consistent with ‘G4EmDNAPhysics_option 5’ (hereinafter referred to as ‘option 5’); secondly, there are differences for the information of track structure and mean lineal energy between ‘option 2’ ‘option 4’ and ‘G4EmDNAPhysics_option 6’ (hereinafter referred to as ‘option 6’); thirdly, the influence of the model on the mean lineal energy decreases with the increase of the radius of the scoring spheres, whereas mean lineal energy increases as the tracking cut increases. Several alternative discrete physics constructors of Geant4-DNA are comprehensively discussed overlaying multiple perspectives under different conditions in this work.

Comparing the Microdosimetric Kinetic Model-Predicted Proton RBE Values Using Constant and Variable Quadratic Beta Coefficients

In the current microdosimetric kinetic model (MKM) model for predicting the biological effects of therapeutic particles, the quadratic beta coefficient is assumed to be a constant independent of radiation types. However, many cell irradiation experiments have shown the variable beta coefficients for different beam qualities. The purpose of this study is to compare the predicted relative biological effectiveness (RBE) of protons using the MKM model with a constant beta value and beam quality (i.e., the microdosimetric quantity lineal energy) dependent beta values. We have spatially mapped the biological effects along the path of a mono-energetic 81.4 MeV proton beam using the high-throughput method. Clonogenic cell survival curves with twelve different beam qualities (dose mean lineal energy, yD) were obtained for the non-small cell lung cancer (NSCLC) cell line H460. The Cs-137 photon source was used as the reference. The microdosimetric data were obtained from the track-structure Monte Carlo simulations using Geant4-DNA with a water sphere (d = 2 μm) as the target. The alpha and beta coefficients for protons and reference photons were obtained from the linear-quadratic fit results of each survival curve. The beta coefficient of photons was selected as the constant beta value for MKM predictions. We calculated the alpha coefficient using the constant beta first and then using the variable beta for each proton beam quality. The MKM model predicted RBE values as a function of beam quality were then compared with the experimental RBE. The twelve evenly sampled yD values of the proton beam ranges from 1.38 to 21.77 keV/μm with the value of 12.93 keV/μm at the Bragg peak. The alpha and beta coefficients from Cs-137 photon source are 0.23 Gy-1 and 0.10 Gy-2. Our experiments have shown a general trend that the alpha and beta values of protons from experimental survival curves increase with the increase of yD. The experimental RBE increases from 0.99 (yD = 1.38 keV/μm) to 2.85 (yD = 21.77 keV/μm) in a rather non-linear way. Using the constant beta based MKM model, the predicted RBE (cell surviving fraction = 0.1) only varies from 1.00 to 1.12, nearly independent of the beam quality. In contrast, using the variable beta based MKM model, the predicted proton RBE increases as a function of yD and varies from 1.00 to 2.73. The relative difference between variable beta based MKM model prediction and the experimental RBE is within ±10%. From the results in this study, we conclude that using the experimentally determined variable beta values, MKM model can better predict the biological effects of protons. However, more experiments for different beam qualities and different cell lines are needed to further consolidate the conclusion made here.

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.

Study of microdosimetric parameters of mixed radiation fields produced by D-T neutron generators of different designs - a comparison

Abstract Microdosimetric parameters measured by a TEPC have been studied in the mixed radiation fields generated by the two D-T neutron generators developed at BARC and IGCAR facilities of DAE, India. These two generators differ in their sizes and designs. The present study reveals that the radiation fields by these two generators are significantly different. In the case of compact design (IGCAR generator) due to the significant contribution of bremsstrahlung photons in the total radiation field, the frequency and dose mean lineal energy, average quality factors are considerably less (factors of 3 to 6) than the BARC D-T generator. The study also reveals that the photon contribution to the total dose equivalent can be as high as 30% for compact D-T generator.

3D sensitive volume microdosimeter with improved tissue equivalency: charge collection study and its application in 12C ion therapy

This research focuses on the characterisation of a new 3D sensitive volume (SV) microdosimeter covered with polyimide – a material which closely mimics human tissue. The electrical and charge collection properties of the device were investigated and its application in 12C ion therapy were studied. Charge collection studies revealed uniform charge collection and no cross talk between adjacent SVs. To study the microdosimetric response in 12C ion therapy, the new polyimide mushroom microdosimeter were placed at various positions along the central axis of a 290 MeV/u 12C ion spread out Bragg peak (SOBP) at the Heavy Ion Medical Accelerator in Chiba (HIMAC), Japan. From these microdosimetric spectra, dose mean lineal energy and RBE10 results were obtained, with RBE10 increasing from 1.3 at the entrance to 2.7 at the end of the SOBP. The results obtained in this work show that the new generation of mushroom microdosimeters, covered with tissue equivalent polyimide material, are a useful tool for quality assurance in heavy ion therapy applications.

MICRODOSIMETRY OF CLINICAL ION BEAMS: CONVERTING SPECTRA FROM DIAMOND SLAB TO WATER OF DIFFERENT SHAPES.

Microdosemeters are frequently used today to specify the radiation quality in the framework of ion-beam therapy. The heterogeneity of the detector shapes and the materials limits the possibility of comparing directly spectra and mean lineal energies. A method was recently studied to convert the spectra obtained with unidirectional ion beams in slab detectors to those obtained with detectors of different in shape and material. The method is based on the observation that the lineal-energy spectra of slab detector, in a restricted energy interval, approximate the Linear Energy Transfer distributions at corresponding material and particle type and energies. In this study, the experimental spectra collected with a slab diamond detector are converted to the spectra that would be obtained using water detectors of spherical and cylindrical shapes.