Graphite Calorimeter Research Articles

Proof-of-Principle of Absolute Dosimetry Using an Absorbed Dose Portable Calorimeter with Laser-Driven Proton Beams

Charged particle beams driven to ultra-high dose rates (UHDRs) have been shown to offer potential benefits for future clinical applications, particularly in the reduction of normal-tissue toxicity. Studies of the so-called FLASH effect have shown promise, generating huge interest in high dose rate radiation studies. With laser-driven proton beams, where the duration of the proton burst delivered to a sample can be as short as hundreds of picoseconds, the instantaneous dose rates are several orders of magnitude higher than those used for conventional radiotherapy. The dosimetry of these beam modalities is not trivial, with conventional active detectors, such as ionisation chambers, experiencing saturation effects making them unusable at the extremely high dose rates. Calorimeters, measuring the radiation-induced temperature rise in an absorber, offer an ideal candidate for the dosimetry of UHDR beams. However, their application in the measurement of laser-driven UHDR beams has so far not been trialled, and their effective suitability to work with the quasi-instantaneous and inhomogeneous dose deposition patterns and the harsh environment of a laser-plasma experiment has not been tested. The measurement of the absorbed dose of laser-driven proton beams was conducted in a first-of-its-kind investigation, employing the VULCAN-PW laser system of the Central Laser Facility (CLF) at the Rutherford Appleton Laboratory (RAL), using a small-body portable graphite calorimeter (SPGC) developed at the National Physical Laboratory (NPL) and radiochromic films. A small number of shots were recorded, with the corresponding absorbed dose measurements resulting from the induced temperature rise. The effect of the electromagnetic pulse (EMP) generated during laser–target interaction was assessed on the system, showing no significant effects on the derived signal-to-noise ratio. These proof-of-principle tests highlight the ability of calorimetry techniques to measure the absorbed dose for laser-driven proton beams.

Monte Carlo modelling of a prototype small-body portable graphite calorimeter for ultra-high dose rate proton beams

Background and purposeAccurate dosimetry in Ultra-High Dose Rate (UHDR) beams is challenging because high levels of ion recombination occur within ionisation chambers used as reference dosimeters. A Small-body Portable Graphite Calorimeter (SPGC) exhibiting a dose-rate independent response was built to offer reduced uncertainty on secondary standard dosimetry in UHDR regimes. The aim of this study was to quantify the effect of the geometry and material properties of the device on the dose measurement. Materials and methodsA detailed model of the SPGC was built in the Monte Carlo code TOPAS (v3.6.1) to derive the impurity and gap correction factors, kimp and kgap. A dose conversion factor, DwMC/DgMC, was also calculated using FLUKA (v2021.2.0). These factors convert the average dose to its graphite core to the dose-to-water for a 249.7 MeV mono-energetic spot-scanned clinical proton beam. The effect of the surrounding Styrofoam on the dose measurement was examined in the simulations by substituting it for graphite. ResultsThe kimp and kgap correction factors were 0.9993 ± 0.0002 and 1.0000 ± 0.0001, respectively when the Styrofoam was not substituted, and 1.0037 ± 0.0002 and 0.9999 ± 0.0001, respectively when substituted for graphite. The dose conversion factor was calculated to be 1.0806 ± 0.0001. All uncertainties are Type A. ConclusionsImpurity and gap correction factors, and the dose conversion factor were calculated for the SPGC in a FLASH proton beam. Separating out the effect of scatter from Styrofoam insulation showed this as the dominating correction factor, amounting to 1.0043 ± 0.0002.

A portable primary-standard level graphite calorimeter for absolute dosimetry in clinical pencil beam scanning proton beams

Objective. To report the use of a portable primary standard level graphite calorimeter for direct dose determination in clinical pencil beam scanning proton beams, which forms part of the recommendations of the proposed Institute of Physics and Engineering in Medicine (IPEM) Code of Practice (CoP) for proton therapy dosimetry. Approach. The primary standard proton calorimeter (PSPC) was developed at the National Physical Laboratory (NPL) and measurements were performed at four clinical proton therapy facilities that use pencil beam scanning for beam delivery. Correction factors for the presence of impurities and vacuum gaps were calculated and applied, as well as dose conversion factors to obtain dose to water. Measurements were performed in the middle of 10 × 10 × 10 cm3 homogeneous dose volumes, centred at 10.0, 15.0 and 25.0 g·cm−2 depth in water. The absorbed dose to water determined with the calorimeter was compared to the dose obtained using PTW Roos-type ionisation chambers calibrated in terms of absorbed dose to water in 60Co applying the recommendations in the IAEA TRS-398 CoP. Main results. The relative dose difference between the two protocols varied between 0.4% and 2.1% depending on the facility. The reported overall uncertainty in the determination of absorbed dose to water using the calorimeter is 0.9% (k = 1), which corresponds to a significant reduction of uncertainty in comparison with the TRS-398 CoP (currently with an uncertainty equal or larger than 2.0% (k = 1) for proton beams). Significance. The establishment of a purpose-built primary standard and associated CoP will considerably reduce the uncertainty of the absorbed dose to water determination and ensure improved accuracy and consistency in the dose delivered to patients treated with proton therapy and bring proton reference dosimetry uncertainty in line with megavoltage photon radiotherapy.

Alanine response to low energy synchrotron x-ray radiation

Objective. The radiation response of alanine is very well characterized in the MV photon energy range where it can be used to determine the dose delivered with an accuracy better than 1%, making it suitable as a secondary standard detector in cancer radiation therapy. This is not the case in the very low energy keV x-ray range where the alanine response is affected by large uncertainties and is strongly dependent on the x-ray beam energy. This motivated the study undertaken here. Approach. Alanine pellets with a nominal thickness of 0.5 mm and diameter of 5 mm were irradiated with monoenergetic x-rays at the Diamond Light Source synchrotron, to quantify their response in the 8–20 keV range relative to 60Co radiation. The absorbed dose to graphite was measured with a small portable graphite calorimeter, and the DOSRZnrc code in the EGSnrc Monte Carlo package was used to calculate conversion factors between the measured dose to graphite and the absorbed dose to water delivered to the alanine pellets. GafChromic EBT3 films were used to measure the beam profile for modelling in the MC simulations. Main results. The relative responses measured in this energy range were found to range from 0.616 to 0.643, with a combined relative expanded uncertainty of 3.4%–3.5% (k = 2), where the majority of the uncertainty originated from the uncertainty in the alanine readout, due to the small size of the pellets used. Significance. The measured values were in good agreement with previously published data in the overlapping region of x-ray energies, while this work extended the dataset to lower energies. By measuring the response to monoenergetic x-rays, the response to a more complex broad-spectrum x-ray source can be inferred if the spectrum is known, meaning that this work supports the establishment of alanine as a secondary standard dosimeter for low-energy x-ray sources.

DEVELOPMENT OF ABSOLUTE MEASUREMENT SYSTEM FOR 𝑾AIR IN 60CO GAMMA RAYS.

In the ICRU Report 90, the uncertainty of the recommendation for ${{W}}_{{air}}$ was changed from 0.15 to 0.35%. The purpose of this study is to develop an absolute measurement system for ${{W}}_{{air}}$ in electrons with lower uncertainty using 60Co gamma rays. ${{W}}_{{air}}$ was determined by the ratio of the energy deposition to the number of ion pairs created in a given volume of air using a graphite calorimeter and a graphite-wall ionisation chamber. The obtained value for ${{W}}_{{air}}$ was 33.91 eV with a relative standard uncertainty of 0.08 eV (k = 1), and was in good agreement with the recommended value of the ICRU Report 90 (33.97 eV). Additionally, the uncertainty for ${{W}}_{{air}}$ obtained in this investigation is 0.23% and is comparable with that reported by Burns etal., which is the lowest uncertainty in recent studies determining ${{W}}_{{air}}$ In the future, the proposed system can be used to measure ${{W}}_{{air}}$ in high-energy electron beams.

Application of a portable primary standard level graphite calorimeter for absolute dosimetry in a clinical low-energy passively scattered proton beam

Objective. A calibration service based on a primary standard calorimeter for the direct determination of absorbed dose for proton beams does not exist. A new Code of Practice (CoP) for reference dosimetry of proton beams is being developed by a working party of the UK Institute of Physics and Engineering in Medicine (IPEM), which will recommend that ionisation chambers are calibrated directly in their clinical beams against the proposed Primary Standard Proton Calorimeter (PSPC) developed at the National Physical Laboratory (NPL). The aim of this work is to report on the use of the NPL PSPC to directly calibrate ionisation chambers in a low-energy passively scattered proton beam following recommendations of the upcoming IPEM CoP. Approach. A comparison between the dose derived using the proposed IPEM CoP and the IAEA TRS-398 protocol was performed, and values were determined experimentally for three types of chambers. In total, 9 plane-parallel and 3 cylindrical chambers were calibrated using the two protocols for two separate visits. Main results. The ratio of absorbed dose to water obtained with the PSPC and with ionisation chambers applying TRS-398 varied between 0.98 and 1.00, depending on the chamber type. The new procedure based on the PSPC provides a significant improvement in uncertainty where absorbed dose to water measured with a user chamber is reported with an uncertainty of 0.9% (1), whereas the TRS-398 protocol reports an uncertainty of 2.0% and 2.3% (1) for cylindrical and plane-parallel chambers, respectively. The values found agree within uncertainties with those from TRS-398 and Monte Carlo calculations. Significance. The establishment of a primary standard calorimeter for the determination of absorbed dose in proton beams combined with the introduction of the associated calibration service following the IPEM recommendations will reduce the uncertainty and improve consistency in the dose delivered to patients.

The probe-format graphite calorimeter, Aerrow, for absolute dosimetry in ultrahigh pulse dose rate electron beams.

The purpose of this investigation is to evaluate the use of a probe-format graphite calorimeter, Aerrow, as an absolute and relative dosimeter of high-energy pulse dose rate (UHPDR) electron beams for in-water reference and depth-dose-type measurements, respectively. In this paper, the calorimeter system is used to investigate the potential influence of dose per pulses delivered up to 5.6Gy, the number of pulses delivered per measurement, and its potential for relative measurement (depth-dose curve measurement). The calorimeter system is directly compared against an Advanced Markus ion chamber. The finite element method was used to calculate heat transfer corrections along the percentage depth dose of a 20-MeV electron beam. Monte Carlo-calculated dose conversion factors necessary to calculate absorbed dose-to-water at a point from the measured dose-to-graphite are also presented. The comparison of Aerrow against a fully calibrated Advanced Markus chamber, corrected for the saturation effect, has shown consistent results in terms of dose-to-water determination. The measured reference depth is within 0.5mm from the expected value from Monte Carlo simulation. The relative standard uncertainty estimated for Aerrow was 1.06%, which is larger compared to alanine dosimetry (McEwen etal. https://doi.org/10.1088/0026-1394/52/2/272) but has the advantage of being a real-time detector. In this investigation, it was demonstrated that the Aerrow probe-type graphite calorimeter can be used for relative and absolute dosimetries in water in an UHPDR electron beam. To the author's knowledge, this is the first reported use of an absorbed dose calorimeter for an in-water percentage depth-dose curve measurement. The use of the Aerrow in quasi-adiabatic mode has greatly simplified the signal readout, compared to isothermal mode, as the resistance was directly measured with a high-stability digital multimeter.

Ultrahigh dose rate pencil beam scanning proton dosimetry using ion chambers and a calorimeter in support of first in-human FLASH clinical trial.

PurposeTo provide ultrahigh dose rate (UHDR) pencil beam scanning (PBS) proton dosimetry comparison of clinically used plane‐parallel ion chambers, PTW (Physikalisch‐Technische Werkstaetten) Advanced Markus and IBA (Ion Beam Application) PPC05, with a proton graphite calorimeter in a support of first in‐human proton FLASH clinical trial.MethodsAbsolute dose measurement intercomparison of the plane‐parallel plate ion chambers and the proton graphite calorimeter was performed at 5‐cm water‐equivalent depth using rectangular 250‐MeV single‐layer treatment plans designed for the first in‐human FLASH clinical trial. The dose rate for each field was designed to remain above 60 Gy/s. The ion recombination effects of the plane‐parallel plate ion chambers at various bias voltages were also investigated in the range of dose rates between 5 and 60 Gy/s. Two independent model‐based extrapolation methods were used to calculate the ion recombination correction factors ks to compare with the two‐voltage technique from most widely used clinical protocols.ResultsThe mean measured dose to water with the proton graphite calorimeter across all the predefined fields is 7.702 ± 0.037 Gy. The average ratio over the predefined fields of the PTW Advanced Markus chamber dose to the calorimeter reference dose is 1.002 ± 0.007, whereas the IBA PPC05 chamber shows ∼3% higher reading of 1.033 ± 0.007. The relative differences in the ks values determined from between the linear and quadratic extrapolation methods and the two‐voltage technique for the PTW Advanced Markus chamber are not statistically significant, and the trends of dose rate dependence are similar. The IBA PPC05 shows a flat response in terms of ion recombination effects based on the ks values calculated using the two‐voltage technique. Differences in ks values for the PPC05 between the two‐voltage technique and other model‐based extrapolation methods are not statistically significant at FLASH dose rates. Some of the ks values for the PPC05 that were extrapolated from the three‐voltage linear method and the semiempirical model were reported less than unity possibly due to the charge multiplication effect, which was negligible compared to the volume recombination effect in FLASH dose rates.ConclusionsThe absolute dose measurements of both PTW Advanced Markus and IBA PPC05 chambers are in a good agreement with the National Physical Laboratory graphite calorimeter reference dose considering overall uncertainties. Both ion chambers also demonstrate good reproducibility as well as stability as reference dosimeters in UHDR PBS proton radiotherapy. The dose rate dependency of the ion recombination effects of both ion chambers in cyclotron generated PBS proton beams is acceptable and therefore, both chambers are suitable to use in clinical practice for the range of dose rates between 5 and 60 Gy/s.

Dose area product primary standards established by graphite calorimetry at the LNE-LNHB for small radiation fields in radiotherapy

PurposeTo present primary standards establishment in terms of Dose Area Product (DAP) for small field sizes. MethodsA large section graphite calorimeter and two plane-parallel ionization chambers were designed and built in-house. These chambers were calibrated in a 6MV FFF beam at the maximum dose rate of 1400 UM/min for fields defined by specifically designed circular collimators of 5, 7.5, 10, 13 and 15 mm diameter and jaws of 5, 7, 10, 13 and 15 mm side length on a Varian TrueBeam linac. ResultsThe two chambers show the same behaviour regardless of field shape and size. From 5 to 15 mm, calibration coefficients slightly increase with the field size with a magnitude of 1.8% and 1.1% respectively for the two chambers, and are independent of the field shape. This tendency was confirmed by Monte Carlo calculations. The average associated uncertainty of the calibration coefficients is around 0.6% at k=1. ConclusionsFor the first time, primary standards in terms of DAP were established by graphite calorimetry for an extended range of small field sizes. These promising results open the door for an alternative approach in small fields dosimetry.

Feasibility of operating a millimeter-scale graphite calorimeter for absolute dosimetry of small-field photon beams in the clinic.

To characterize and build a cylindrically layered graphite calorimeter the size of a thimble ionization chamber for absolute dosimetry of small fields. This detector has been designed in a familiar probe format to facilitate integration into the clinical workflow. The feasibility of operating this absorbed dose calorimeter in quasi-adiabatic mode is assessed for high-energy accelerator-based photon beams. This detector, herein referred to as Aerrow MK7, is a miniaturized version of a previously validated aerogel-insulated graphite calorimeter known as Aerrow. The new model was designed and developed using numerical methods. Medium conversion factors from graphite to water, small-field output correction factors, and layer perturbation factors for this dosimeter were calculated using the EGSnrc Monte Carlo code system. A range of commercially available aerogel densities were studied for the insulating layers, and an optimal density was selected by minimizing the small-field output correction factors. Heat exchange within the detector was simulated using a five-body compartmental heat transfer model. In quasi-adiabatic mode, the sensitive volume (a 3mm diameter cylindrical graphite core) experiences a temperature rise during irradiation on the order of 1.3mK·Gy-1 . The absorbed dose is obtained by calculating the product of this temperature rise with the specific heat capacity of the graphite. The detector was irradiated with 6MV ( =63.5%) and 10MV ( =71.1%) flattening filter-free (FFF) photon beams for two field sizes, characterized by dimensions of 2.16 and 11.0cm. The dose readings were compared against a calibrated Exradin A1SL ionization chamber. All dose values are reported at in water. The field output correction factors for this dosimeter design were computed for field sizes ranging from =0.54 to 11.0cm. For all aerogel densities studied, these correction factors did not exceed 1.5%. The relative dose difference between the two dosimeters ranged between 0.3% and 0.7% for all beams and field sizes. The smallest field size experimentally investigated, =2.16cm, which was irradiated with the 10MV FFF beam, produced readings of 84.4cGy (±1.3%) in the calorimeter and 84.5cGy (±1.3%) in the ionization chamber. The median relative difference in absorbed dose values between a calibrated A1SL ionization chamber and the proposed novel graphite calorimeter was 0.6%. This preliminary experimental validation demonstrates that Aerrow MK7 is capable of accurate and reproducible absorbed dose measurements in quasi-adiabatic mode.

Changes of KRISS primary standards by implementing ICRU 90 recommendation

The International Commission of Radiation Units and Measurements (ICRU) published report No. 90 (i.e., Key Data for Ionizing Radiation Dosimetry: Measurement Standards and Applications) in 2016. The present study explores the impact of ICRU 90 on the primary standards of the Korea Research Institute of Standards and Science (KRISS). To this end, first, two new correction factors (kii and kw) for the free-air ionization chambers were introduced; it influences the standards related to kV X-ray beams. The new correction factors were calculated by simulating the initial electrons’ generations using the GEANT4 code. The second recommendation considered was for changes in the mean excitation energies of graphite and water, and the use of the crystalline density of graphite in the stopping power evaluation. These recommendations should be adopted for the graphite-walled cavity chamber and graphite calorimeter, which are the primary standards of the KRISS for air kerma in gamma rays and absorbed dose to water in high-energy photons, respectively. EGSnrc code-based evaluations of the electron stopping-power ratios for gamma rays were performed for the KRISS graphite-walled cavity chamber. Regarding graphite calorimetry in high-energy photon beams, the recommendations on graphite and water have been studied and presented in a previous paper. The third consideration is the recommendation for increasing uncertainty of the mean energy of electrons expended in dry air per ion pair production, $${\left(W/e\right)}_{\mathrm{air}}$$ , from 0.15% to 0.35%. The impact of the increased uncertainty under the conditions of the use of the existing components was summarized, together with the overall changes to the KRISS primary standards related to air kerma. Consequently, the difference between the products of the calculated kii and kW for the free-air ionization chamber standard and the corresponding values given in the ICRU 90 report was less than 0.10% for mono-energetic photons in the range of 1–400 keV. Regarding the X-ray beams recommended by the Consultative Committee for Ionizing Radiation (CCRI), the calculated correction factors deviated from those of the Bureau International des Poids et Mesures (BIPM) standards by 0.03%. In the case of the graphite-walled cavity chamber, changes of 0.65–0.77% were observed for the gamma-ray air kerma standards. The observed changes were reasonable, considering that similar changes were found in those reported by other research groups. Regarding the combined uncertainties, the uncertainties for the air kerma primary standards increased up to approximately 0.40%, primarily because of $${\left(W/e\right)}_{air}$$ . The changes to the KRISS primary standards that were observed in the present study will be adopted for the measurements performed since 2021.

Operating a graphite calorimeter in quasi-isothermal mode under high-energy x-ray beams

In this study, we developed a semi-active method to run a graphite calorimeter in the quasi-isothermal mode under high-energy x-ray beams. The rate of energy imparted by the beam during irradiation was compensated mainly by removing the electrical heating power based on the pre-calculation and in part by an active automated algorithm, as well, while the temperature of the calorimeter core was kept constant. Irradiations were performed under the linear electron accelerator x-ray beams at 6, 8, 10, 15, and 18 MV. A simple model was applied to analyze the results. The energy imparted to the core was determined with an uncertainty level of 0.2%–0.3%, and the results were reaffirmed by comparing it with that obtained by the quasi-adiabatic mode. The normalized root-mean-square deviation to the mean from the quasi-adiabatic mode was 0.11%, and the associated uncertainty was 0.16% taking into account the correlation of the uncertainty components. This level of agreement showed that the present method is practical for the high-energy x-ray dosimetry.

Using a small-core graphite calorimeter for dosimetry and scintillator quenching corrections in a therapeutic proton beam

Organic plastic scintillation detectors (PSDs) are known to produce less light per absorbed dose in highly dense radiations in comparison with e.g. 60Co gamma beams. This so-called ionization density quenching can be experimentally determined by comparison of the scintillator output with the absorbed dose established with a reference detector. The hypothesis of this work was that a newly developed small-core graphite calorimeter (core size: ø5mm × 7mm) can be used as reference for such measurements. The potential benefit of a calorimetric reference would be to have a robust and accurate reference with well-understood dosimetry properties even in high-intensity FLASH beams. As a first step, the hypothesis was tested by comparing previously established quenching parameter estimates for the BCF-60 scintillating material with data obtained with the new instrument at different depths along the central axis of a 170 MeV scanned proton beam. After the calorimetric measurements, scintillator measurements were acquired under equivalent conditions by positioning the PSD in a replica graphite core nominally identical to the core used for calorimetry. To experimentally document details of the irradiations, the spot width was mapped along the central beam axis using a new technique based on a PSD and a time-to-distance conversion procedure. Analysing the proton data in the framework of the Birks model, the graphite calorimeter gave a quenching parameter for BCF-60 in agreement with literature values. The consistency between the calorimetric results and the other sources of information supports the validity of the new method, and we therefore aim to apply it for characterization of other detectors in more intense beams where ionometry cannot serve as reference.

Performance and Characterization of a Neutral Beam Propulsion Technology Demonstrator

A neutral beam propulsion technology demonstrator has been fabricated and tested. It uses gas diffusion neutralization to create a globally neutral plasma from an ion source. Thrust measurements for the neutral beam demonstrate that thrust at the very low levels produced by this system can be measured during thermal drift of the variable amplitude hanging pendulum with extended range thrust stand. The beam energy, which exhibits good quantitative agreement with the thrust measurements, is measured as a function of neutralizer gas flow rate using a graphite calorimeter. A Faraday cup is used to measure the neutralization of the beam. These measurements generally exhibit quantitative agreement with predictive analytical and numerical models. Losses owing to the use of a grounded metal neutralizer are discussed, and it is quantitatively demonstrated through thrust, neutralization, and calorimetry measurements that the neutralizer is the source of the performance losses.

Monte Carlo studies on dose conversion factors from graphite to water for high energy X-ray beams

An evaluation of the dose conversion factor from graphite to water is essential to measure the absorbed dose to water using a graphite calorimeter. However, it is difficult to obtain the conversion factors from measurements. In the present study, Monte Carlo (MC) studies were performed to evaluate the absorbed dose conversion factor from graphite to water. Based on the Electron Gamma Shower MC codes from National Research Council Canada, the modeling of the medical linear accelerator gantry at the Korea Research Institute of Standards and Science (KRISS) was optimized to obtain phase space files for evaluation of the absorbed dose conversion factor from graphite to water. The evaluated dose conversion factors for the KRISS graphite calorimeter were in the range of 1.040–1.085 for beams of 6–25 MV. A linear correlation between the dose conversion factors and tissue phantom ratio in water at depths of 20 cm and 10 cm (TPR20,10) of each X-ray beam was observed. To understand sensitivity of the obtained dose conversion factors, case studies were performed under different conditions in the simulation, such as different electron stopping power, assumed graphite bulk density, and different MC code. The uncertainties of the dose conversion factors will be finalized considering the results of sensitivity studies in the present study and additional studies in the future.

Absolute dosimetry of a 1.5 T MR-guided accelerator-based high-energy photon beam in water and solid phantoms using Aerrow.

In this work, the fabrication, operation, and evaluation of a probe-format graphite calorimeter - herein referred to as Aerrow - as an absolute clinical dosimeter of high-energy photon beams while in the presence of a B=1.5T magnetic field is described. Comparable to a cylindrical ionization chamber (IC) in terms of utility and usability, Aerrow has been developed for the purpose of accurately measuring absorbed dose to water in the clinic with a minimum disruption to the existing clinical workflow. To our knowledge, this is the first reported application of graphite calorimetry to magnetic resonance imaging (MRI)-guided radiotherapy. Based on a previously numerically optimized and experimentally validated design, an Aerrow prototype capable of isothermal operation was constructed in-house. Graphite-to-water dose conversions as well as magnetic field perturbation factors were calculated using Monte Carlo, while heat transfer and mass impurity corrections and uncertainties were assessed analytically. Reference dose measurements were performed in the absence and presence of a B=1.5T magnetic field using Aerrow in the 7MV FFF photon beam of an Elekta MRI-linac and were directly compared to the results obtained using two calibrated reference-class IC types. The feasibility of performing solid phantom-based dosimetry with Aerrow and the possible influence of clearance gaps is also investigated by performing reference-type dosimetry measurements for multiple rotational positions of the detector and comparing the results to those obtained in water. In the absence of the B-field, as well as in the parallel orientation while in the presence of the B-field, the absorbed dose to water measured using Aerrow was found to agree within combined uncertainties with those derived from TG-51 using calibrated reference-class ICs. Statistically significant differences on the order of (2-4)%, however, were observed when measuring absorbed dose to water using the ICs in the perpendicular orientation in the presence of the B-field. Aerrow had a peak-to-peak response of about 0.5% when rotated within the solid phantom regardless of whether the B-field was present or not. This work describes the successful use of Aerrow as a straightforward means of measuring absolute dose to water for large high-energy photon fields in the presence of a 1.5T B-field to a greater accuracy than currently achievable with ICs. The detector-phantom air gap does not appear to significantly influence the response of Aerrow in absolute terms, nor does it contribute to its rotational dependence. This work suggests that the accurate use of solid phantoms for absolute point dose measurement is possible with Aerrow.

Determination of Wair in high-energy electron beams using graphite detectors.

ICRU Report 90 on Key Data for Ionizing-Radiation Dosimetry: Measurement Standards and Applications (2014) has reaffirmed the recommended value of the mean energy required to create an ion pair in air, Wair , to be 33.97(12)eV. The report also indicates that this "constant" of radiation dosimetry is energy independent above 10keV, since there is no theoretical or experimental evidence to the contrary. The goal of this investigation is to obtain additional experimental determinations of Wair in high energy beams and thus to verify the suggested energy independence. Wair can be evaluated by combining ionometric and calorimetric measurements with a calculated ratio of the absorbed dose in the ion chamber air cavity and that of the calorimeter absorbing element. In this investigation, a graphite parallel plate chamber and a graphite calorimeter were used and the dose ratio was calculated using the EGSnrc Monte Carlo code. Measurements were made in electron beams from the NRC Vickers linear accelerator at two incident energies, 20 and 35MeV. A range of average energies at the measurement point were obtained by inserting graphite plates in the primary beam. The average value of Wair obtained in this investigation is 33.85(18)eV which is consistent with the recommended value of 33.97(12)eV where the number in brackets represents the combined standard uncertainty of the value, referring to the corresponding last digits. The individual values of Wair do not show any statistically significant energy dependence. The overall combined uncertainty of 0.5% meets the original target of the investigation. A larger-scale investigation, involving more individual energy points and a wider range of electron energies is required to go further and, for example, comment on the Wair energy dependency question raised by Tessier et al. [Med. Phys. 2018;45:370-381].

Characterisation of a megavolt X-Ray therapeutic beam using non-orthogonal strip detectors for future enhanced primary standard dosimetry

The National Physical Laboratory (NPL) provides traceable high accuracy measurements for absorbed dose via primary standard graphite calorimeters. Patients undergoing radiotherapy for cancer treatments in the U.K. receive a dose that is traceable to these devices. In the case of hadron radiotherapy, reducing the uncertainty of these measurements requires the addition of spatial information to understand not only the integral dose delivered, but where the energy is deposited locally. The PRaVDA consortium developed strip detectors able to track individual protons at low rates for proton computed tomography. For calorimetry at clinical dose rates only real-time spot location is required, which could be projected to reconstruct a 2D visualisation from which the beam centre can be determined. This work reports on a proof-of-concept trial carried out using 6 MV X-rays using an Elekta Synergy linear accelerator. We show how the beam positions can be determined within ± 0.5 mm of the expected radiation beam position, which is similar to the tolerances of the field defining multileaf collimator within the linear accelerator treatment head. The trial will be extended to perform measurements in a clinical proton beam alongside the NPL graphite proton calorimeter.

Complementary dosimetry for a 6 MeV electron beam

Abstract Two methods for inferring the dose of the electron beam (EB) provided by the ALID-7 linear accelerator (LINAC) are compared. The main parameters of the electron beam are: energy 6 M e V , pulse duration 4 μs and frequency 53 H z . The reference dosimeter is a graphite calorimeter built in-house, whose measured dose depends on the irradiation time. In order to modify the dose for a given irradiation time, the voltage applied on the filament of the electron gun is varied in the range 8–12 V. The calorimeter can provide reliable measurements (doses above 0.48 k G y ) for irradiation times sufficiently long (above 10 s) and for filament voltages where stable operation of the LINAC is achieved (above 9 V). The measured dose is compared with that deduced from measurements of the beam fluence given by a Faraday cup (FC), by taking into account the stopping power features of the electron beam into graphite. The electron beam charge per pulse measured at the exit of the LINAC is between 3.7 × 10−9 C at 8 V, and 2.4 × 10−7 C at 12 V. A good match is obtained particularly at 12 V, which is the typical operating filament voltage.

Dose detectors, sensors, and their applications

This article is intended to present the different types of particle and radiation detectors available for applications in particle therapy. Several types of detectors and sensors exist for measurements of absorbed dose in reference and nonreference conditions and the ones in use for beam monitoring. Therefore, this manuscript focuses on the following applications: (a) primary methods for dosimetry in ion beams, (b) measurements of absorbed dose in realistic patient-specific scenarios with different dose delivery configurations, and (c) beam monitoring. Water and graphite calorimeters, Faraday cups, ionization based gas detectors are described first, followed by the description of the protocols for proton and ion dosimetry. Then films, scintillator and solid-state detectors are reviewed. Finally, several types of ionization chambers (large, pinpoint, arrays of chambers arranged in regular 1D or 2D pattern, parallel-plate configuration with large integral electrodes or with anode segmented in strips or pixels, multi-wire, and the multi-gap prototype) have been considered. New beam monitors to deal with a wide range of intensity and pulsed beams expected from new accelerators, different dose fractionation and advanced delivery techniques are presented. The existing detectors available for particle therapy have been described taking into account the different requirements for devices used in reference and nonreference conditions and the ones designed for beam monitoring.