Measurements Of Linear Energy Transfer Research Articles

Linear Energy Transfer Measurements and Estimation of Relative Biological Effectiveness in Proton and Helium Ion Beams Using Fluorescent Nuclear Track Detectors

PurposeTo develop a methodology for assessing the linear energy transfer (LET) and relative biological effectiveness (RBE) in clinical proton and helium-ion beams using fluorescent nuclear track detectors (FNTDs). Methods and materialsFNTDs were exposed behind solid water to proton and helium- (4He-) ion spread-out Bragg peaks (SOBPs). Detectors were imaged with a confocal microscope, and the LET spectra were derived from the fluorescence intensity. The track- and dose-averaged LET (LETF and LETD, respectively) were calculated from the LET spectra. LET measurements were used as input on RBE-models to estimate the RBE. Human alveolar adenocarcinoma cells (A549) were exposed at the same positions as the FNTDs. The RBE was calculated from the resulting survival curves. All measurements were compared with Monte Carlo simulations. ResultsFor protons, average relative differences between measurements and simulations were 6% and 19% for LETF and LETD, respectively. For helium ions the same differences were 11% for both quantities. The position of the experimental LET spectra primary peaks agreed with the simulations within 9% and 14% for protons and helium-ions, respectively. For the RBE-models using LETD as input, FNTD-based RBE values ranged from 1.02 ± 0.01 to 1.25 ± 0.04 and from 1.08 ± 0.09 to 2.68 ± 1.26 for protons and helium-ions, respectively. The average relative differences between these values and simulations were 2% and 4%. For A549 cells, the RBE ranged from 1.05 ± 0.07 to 1.47 ± 0.09 and from 0.89 ± 0.06 to 3.28 ± 0.20 for protons and helium-ions, respectively. Regarding the RBE-weighted dose (2.0 Gy at the SOBP), the differences between simulations and measurements were below 0.10 Gy. ConclusionThis study demonstrates for the first time that FNTDs can be used to perform direct LET measurements and to estimate the RBE in clinical proton and helium-ion beams.

Clinical dose assessment for scanned carbon-ion radiotherapy using linear energy transfer measurements and Monte Carlo simulations

Objective. Dosimetric commissioning of treatment planning systems (TPS) focuses on validating the agreement of the physical dose with experimental data. For carbon-ion radiotherapy, the commissioning of the relative biological effectiveness (RBE) is necessary to predict the clinical outcome based on the radiation quality of the mixed radiation field. In this study, we proposed a approach for RBE commissioning using Monte Carlo (MC) simulations, which was further strengthen by RBE validation based on linear energy transfer (LET) measurements. Approach. First, we tuned the MC simulation based on the results of dosimetric experiments including the beam ranges, beam sizes, and MU calibrations. Furthermore, we compared simulated results to measured depth- and radial-LET distributions of the 430 MeV u−1 carbon-ion spot beam with a 1.5 mm2, 36 μm thick silicon detector. The measured dose-averaged LET (LETd) and RBE were compared with the simulated results. The RBE was calculated based on the mixed beam model with linear-quadratic parameters depending on the LET. Finally, TPS-calculated clinical dose profiles were validated through the tuned MC-based calculations. Main results. A 10 keV μm−1 and 0.15 agreement for LETd and RBE, respectively, were found between simulation and measurement results obtained for a 2σ lateral size of 430 MeV u−1 carbon-ion spot beam in water. These results suggested that the tuned MC simulation can be used with acceptable precision for the RBE and LET calculations of carbon-ion spot beam within the clinical energy range. For physical and clinical doses, the TPS- and MC-based calculations showed good agreements within 1.0% at the centre of the spread-out Bragg peaks. Significance. The tuned MC simulation can accurately reproduce the actual carbon-ion beams, and it can be used to validate the physical and clinical dose distributions calculated by TPS. Moreover, the MC simulation can be used for dosimetric commissioning, including clinical doses, without LET measurements.

Measurements of linear energy transfer (LET) distributions by CR-39 for a therapeutic carbon ion beam with a new 2D ripple filter

This study aims to measure the linear energy transfer (LET) spectra of therapeutic carbon ion beams with a new 2D 6 mm thick ripple filter (RiFi) by using a CR-39 detector. A water phantom was irradiated by three different single energy carbon beams (86.2 MeV/u, 276.5 MeV/u, and 430.0 MeV/u) at Shanghai Proton and Heavy Ion Center (SPHIC), CR-39 detectors were used to measure the LET spectra with and without the RiFi. The Monte Carlo toolkit Geant4 was employed to perform the simulations. The dose-averaged LET (LETd) was calculated according to the LET spectra. The results showed that the 2D RiFi broadens the Bragg peak width and LETd distribution. The simulation LET spectra of primary carbon ions were in good agreement with the experimental results. The LETd can be well estimated by using the measured LET spectra. This result indicated that the RiFi affects the depth-dose distribution and the shape of LET spectra for mono-energetic carbon beams. The CR-39 detector is suitable for LET spectra measurement. It is hoped that this work could provide a new reference method for the cross-validation of basic data in carbon ion clinical quality assurance and benefit clinical treatment.

Ion species discrimination method by linear energy transfer measurement in Fujifilm BAS-SR imaging plate.

We have developed a novel discrimination methodology to identify ions in multispecies beams with similar charge-to-mass ratios, but different atomic numbers. After an initial separation by charge-to-mass ratios using co-linear electric and magnetic fields, individual ions can be discriminated by considering the linear energy transfer of ions irradiating a stimulable phosphor plate (Fujifilm imaging plate) by comparison with the Monte Carlo calculation. We apply the method to energetic multispecies laser-driven ion beams and use it to identify silver ions produced by the interaction between a high contrast, high intensity laser pulse; and a sub-micrometer silver foil target. We also show that this method can be used to calibrate the imaging plate for arbitrary ion species in the range of Z ≥ 6 with dE/dx > 0.1 MeV/μm without requiring individual calibration.

Silicon equivalent gas in silicon equivalent proportional counter - Monte Carlo simulations

Single event effects of secondary particles produced in silicon by proton and neutron irradiations were studied using a silicon equivalent proportional counter (SEPC), constructed for the measurements of linear energy transfer (LET). For optimal measurements, a silicon equivalent gas in the cavity of SEPC was needed. In the present work, Monte Carlo Geant4 simulations were performed to analyse the LET spectra and the secondary particle yields in several candidate gases of a silicon equivalent gas. Among all candidate gases, including propane (C3H8), carbon dioxide (CO2), carbon tetrachloride (CCl4) and noble gases, CCl4 gas seemed the best choice of a silicon equivalent gas.

Calibration of the Al2O3:C optically stimulated luminescence (OSL) signal for linear energy transfer (LET) measurements in therapeutic proton beams

Optically stimulated luminescence (OSL) detectors (OSLDs) have shown potential for measurements of linear energy transfer (LET) in proton therapy beams. However, the technique lacks the efficiency needed for clinical implementation, and a faster, simpler approach to LET measurements is desirable. The goal of this work was to demonstrate and evaluate the potential of calibrating Al2O3:C OSLDs for LET measurements using new methods. We exposed batches of OSLDs to unmodulated proton beams of varying LET and calibrated three parameters of the resulting OSL signals as functions of fluence-averaged LET (ϕ-LET) and dose-averaged LET (D-LET). These three parameters included the OSL curve shape evaluated under continuous wave stimulation (CW-OSL), the OSL curve shape evaluated under pulsed stimulation (P-OSL), and the intensity ratio of the two main emission bands in the Al2O3:C OSL emission spectrum (ultraviolet [UV]/blue ratio). To test the calibration, we then irradiated new batches of OSLDs in modulated proton beams of varying LET, and used the OSL signal parameters to calculate ϕ-LET and D-LET under these new test conditions. Using the P-OSL curve shape, D-LET was measured within 5.7% of the expected value. We conclude that from a single 10 s readout (following initial calibration), both the absorbed dose and LET in proton therapy beams can be measured using OSLDs. This has potential future applications in the quality assurance of proton therapy treatment plans, particularly for those that may account for LET or relative biological effectiveness in their optimization. The methods demonstrated in this work may also be applicable to other particle therapy beams, including carbon ion beams.

WE‐D‐17A‐06: Optically Stimulated Luminescence Detectors as ‘LET‐Meters’ in Proton Beams

Purpose:To demonstrate and evaluate the potential of optically stimulated luminescence (OSL) detectors (OSLDs) for measurements of linear energy transfer (LET) in therapeutic proton beams.Methods:Batches of Al2O2:C OSLDs were irradiated with an absorbed dose of 0.2 Gy in un‐modulated proton beams of varying LET (0.67 keV/μm to 2.58 keV/μm). The OSLDs were read using continuous wave (CW‐OSL) and pulsed (P‐OSL) stimulation modes. We parameterized and calibrated three characteristics of the OSL signals as functions of LET: CW‐OSL curve shape, P‐OSL curve shape and the ratio of the two OSL emission band intensities (ultraviolet/blue ratio). Calibration curves were created for each of these characteristics to describe their behaviors as functions of LET. The true LET values were determined using a validated Monte Carlo model of the proton therapy nozzle used to irradiate the OSLDs. We then irradiated batches of OSLDs with an absorbed dose of 0.2 Gy at various depths in two modulated proton beams (140 MeV, 4 cm wide spread‐out Bragg peak (SOBP) and 250 MeV, 10 cm wide SOBP). The LET values were calculated using the OSL response and the calibration curves. Finally, measured LET values were compared to the true values determined using Monte Carlo simulations.Results:The CW‐OSL curve shape, P‐OSL curve shape and the ultraviolet/blue‐ratio provided proton LET estimates within 12.4%, 5.7% and 30.9% of the true values, respectively.Conclusion:We have demonstrated that LET can be measured within 5.7% using Al2O3:C OSLDs in the therapeutic proton beams used in this investigation. From a single OSLD readout, it is possible to measure both the absorbed dose and LET. This has potential future applications in proton therapy quality assurance, particularly for treatment plans based on optimization of LET distributions.This research was partially supported by the Natural Sciences and Engineering Research Council of Canada.

Linear energy transfer dependence of Al2O3:C optically stimulated luminescence detectors exposed to therapeutic proton beams

Currently, there are no radiation detectors that can be used for routine measurements of linear energy transfer (LET) in particle therapy clinics. In this work, we characterized the LET dependence of Al2O3:C optically stimulated luminescence (OSL) detectors (OSLDs) exposed to therapeutic proton beams in order to evaluate their potential for clinical LET measurements. We evaluated OSLDs that were irradiated with an absorbed dose to water of 0.2 Gy in therapeutic proton beams with average energies ranging between approximately 25 MeV and 200 MeV, resulting in LET in water values between 0.45 and 2.29 keV/μm. We examined two properties of the OSL emission signal in terms of LET dependence: the signal intensities of the blue and ultraviolet (UV) emission bands, and the shapes of the OSL curves. We found that the signal intensity of the UV emission band increased consistently with LET within the range investigated, whereas the intensity of the blue emission band remained constant. Our results also demonstrated that the OSL curve shapes were more LET dependent for signals containing both the blue and UV emission bands than for signals containing only one of the bands. Both metrics we examined in this study – the relative UV/blue emission signal intensities and OSL curve shapes – show potential for LET detection in proton therapy.

Liquid ionization chamber initial recombination dependence on LET for electrons and photons

The possibility of indirect measurements of linear energy transfer (LET) with a liquid ionization chamber (LIC) has been investigated by studying initial recombination losses at different applied voltages. A linear fit is made to the voltage–signal curve and the intersection point of the fit and the voltage-axis is shown to correlate with LET. The LIC applied voltages were 100–700 V, which corresponds to electric field strengths between 0.3 and 2.0 MV m–1. Several different photon and electron beams have been studied, and by using MCNPX™ the respective LET spectra have been determined. The beam qualities in this study were found to have a fluence averaged LET between 0.17 and 1.67 keV µm–1 and a corresponding dose averaged LET between 0.97 and 4.62 keV µm–1. For the experimental data in this study the linear fit method yields consistent results with respect to Monte Carlo simulated LET values. A calibration curve for LET determination is provided for the LIC used in the present work.

Dosimetry for Neutrons from 0.25 to 15 MeV by the Measurement of Linear Energy Transfer Distributions for Secondary Charged Particles in CR-39 Plastic

In the radiation fields of high energy accelerator facilities, high-altitude aircraft and space flights, high-energy neutron dosimetry of ∼20 MeV or more is a significant issue for radiological protection. We studied the feasibility of experimental measurements of linear energy transfer (LET) distributions for secondary charged particles induced by fast neutrons using CR-39 plastic nuclear track detectors. In order to investigate a method of analyzing the CR-39 detectors that is appropriate for fast neutron dosimetry, two-layer CR-39 stacks were exposed to monochromatic neutrons (0.25, 0.55, 5, and 15 MeV) at the Fast Neutron Laboratory of Tohoku University in Japan. We also conducted Monte Carlo calculations to estimate the detection efficiency of the CR-39 detector for recoil protons. The CR-39 detectors treated by single-step chemical etching were used to obtain LET distributions for LET > 10 keV/µm-water. The results indicated that measurements of short-range particles are very important for obtaining the correct LET distributions. Using the measured LET distributions, we calculated neutron sensitivities, absorbed doses and dose equivalents based on the ICRP 60 Q–L relation and averaged quality factors. The dose equivalents were compared with the neutron fluence-to-dose equivalent conversion factors given by ICRP 74 and the averaged quality factors were compared with weighting factors given by ICRP 60 and ICRP 92.

Experimental and calculated LET distributions in the Cosmos-2044 biosatellite orbit.

During the flight of the Cosmos-2044 biosatellite, joint U.S.S.R.-U.S.A. investigations of different characteristics of cosmic radiation (CR) in the near-Earth environment were carried out. The U.S. dielectric track detectors CR-39 and Soviet BYa- and BR-type nuclear photo-emulsions were used as detectors. The present work shows some results of experimental measurements of linear energy transfer (LET) spectra of CR particles obtained with the use of these detectors, which were placed both inside and outside the satellite. The LET spectra measurement with plastic detectors is composed of two parts: the measurement of galactic cosmic rays (GCR) particles, and of short-range particles. The contributions of these components to the total LET distribution at various thicknesses of the shielding were analyzed and the results of these studies are presented. Calculated LET spectra in the Cosmos-2044 orbit were compared with experimental data. On the basis of experimental and calculated values of the LET spectra, absorbed and equivalent CR doses were calculated. In the shielding range of 1-1.5 g cm-2, outside the spacecraft, the photo-emulsions yielded 10.3 mrad d-1 and 27.5 mrem d-1 (LET > or = 2 MeV cm-1) while the CR-39 yielded averages of 1.43 mrad d-1 and 13.4 mrem d-1 (LET > or = 40 MeV cm-1). Inside the spacecraft (> or = 10 g cm-2) the photo-emulsions yielded 8.9 mrad d-1 and 14.5 mrem d-1.