Farmer-type Ionization Chamber Research Articles

Comprehensive characterisation of the IBA myQA SRS for SRS and SBRT patient specific quality assurance.

The myQA SRS (IBA) is a new to market 2D complementary metal oxide semiconductor detector array with an active area 140 × 120mm2 and 0.4mm resolution, making it a potential real-time dosimetry alternative to radiochromic film for stereotactic plan verification. Characterisation of the device was completed to assess performance. The dosimetric properties of the device were assessed for 6FF and 6FFF beams from a Varian TrueBeam STx with high definition multileaf collimator. Clinical suitability of the device for Patient Specific Quality Assurance was verified using ten SRS/SBRT plans, compared against other detectors, as well as multi leaf collimator (MLC) tests including picket fence and chair. Gamma analysis was performed using myQA software with criteria of 4%/1mm. The device demonstrated compliance with recommended specifications for basic tests. After the required warm-up period, the maximum deviation in detector signal from initial readings was 0.2%. Short-term and long-term reproducibility was 0.1% (6FF) and 1.0% (6FFF), respectively. Dose linearity was within 0.3% (6FF) and 0.7% (6FFF) and dose-rate dependence within 1.7% (6FF) and 2.9% (6FFF) and were verified with a Farmer type ionization chamber (PTW 30013). Angular dependence was quantified for coplanar and non-coplanar situations. Output factors and beam profiles measured on the device showed agreement within 1% of baseline RAZOR diode (IBA) and CC04 ionisation chamber (IBA) measurements for field sizes 1 × 1 to 10 × 10cm2. The minimum gamma (4%/1mm) pass rates for MLC-pattern tests were 96.5% and 98.1% for the myQA SRS and film, respectively. The average gamma (4%/1mm) pass rates for SBRT and SRS plans were 98.8% and 99.8% respectively. This work represents one of the first studies performed on the commissioning and performance characterisation of this novel device, demonstrating its accuracy and reliability, making it highly useful as a film alternative in stereotactic treatment plan verification.

Monte Carlo-calculated beam quality and perturbation correction factors validated against experiments for Farmer and Markus type ionization chambers in therapeutic carbon-ion beams

Objective. In current dosimetry protocols, the estimated uncertainty of the measured absorbed dose to water D w in carbon-ion beams is approximately 3%. This large uncertainty is mainly contributed by the standard uncertainty of the beam quality correction factor k Q. In this study, the k Q values in four cylindrical chambers and two plane-parallel chambers were calculated using Monte Carlo (MC) simulations in the plateau region. The chamber-specific perturbation correction factor P of each chamber was also determined through MC simulations. Approach. k Q for each chamber was calculated using MC code Geant4. The simulated k Q ratios in subjected chambers and reference chambers were validated through comparisons against our measured values. In the measurements in Heavy-Ion Medical Accelerator in Chiba, k Q ratios were obtained from D w values of 60Co, 290- and 400 MeV u−1 carbon-ion beams that were measured with the subjected ionization chamber and the reference chamber. In the simulations, f Q (the product of the water-to-air stopping power ratio and P) was acquired from D w and the absorbed dose to air calculated in the sensitive volume of each chamber. k Q values were then calculated from the simulated f Q and the literature-extracted W air and compared with previous publications. Main results. The calculated k Q ratios in the subjected chambers to the reference chamber agreed well with the measured k Q ratios. The k Q uncertainty was reduced from the current recommendation of approximately 3% to 1.7%. The P values were close to unity in the cylindrical chambers and nearly 1% above unity in the plane-parallel chambers. Significance. The k Q values of carbon-ion beams were accurately calculated in MC simulations and the k Q ratios were validated through ionization chamber measurements. The results indicate a need for updating the current recommendations, which assume a constant P of unity in carbon-ion beams, to recommendations that consider chamber-induced differences.

Monte Carlo simulation for proton dosimetry in magnetic fields: Fano test and magnetic field correction factorskBfor Farmer-type ionization chambers.

Objective. In this contribution we present a special Fano test for charged particles in presence of magnetic fields in the MC code TOol for PArticle Simulation (TOPAS), as well as the determination of magnetic field correction factorskBfor Farmer-type ionization chambers using proton beams.Approach. Customized C++ extensions for TOPAS were implemented to model the special Fano tests in presence of magnetic fields for electrons and protons. The Geant4-specific transport parameters,DRoverRandfinalRange,were investigated to optimize passing rate and computation time. ThekBwas determined for the Farmer-type PTW 30013 ionization chamber, and 5 custom built ionization chambers with same geometry but varying inner radius, testing magnetic flux density ranging from 0 to 1.0 T and two proton beam energies of 157.43 and 221.05 MeV.Main results. Using the investigated parameters, TOPAS passed the Fano test within 0.39 ± 0.15% and 0.82 ± 0.42%, respectively for electrons and protons. The chamber response (kB,M,Q) gives a maximum at different magnetic flux densities depending of the chamber size, 1.0043 at 1.0 T for the smallest chamber and 1.0051 at 0.2 T for the largest chamber. The local dose differencecBremained ≤ 0.1% for both tested energies. The magnetic field correction factorkB, for the chamber PTW 30013, varied from 0.9946 to 1.0036 for both tested energies.Significance. The developed extension for the special Fano test in TOPAS MC code with the adjusted transport parameters, can accurately transport electron and proton particles in magnetic field. This makes TOPAS a valuable tool for the determination ofkB. The ionization chambers we tested showed thatkBremains small (≤0.72%). To the best of our knowledge, this is the first calculations ofkBfor proton beams. This work represents a significant step forward in the development of MRgPT and protocols for proton dosimetry in presence of magnetic field.

Quality assurance of six cylindrical and two parallel plate chambers by radioactive check device: Influence of chamber age to its performance

PurposeTo evaluate the performances of eight ionization chambers, (two types) manufactured in a time span of 14 years, with radioactive source devices and photon beams. MethodsThe measurements were performed with six Farmer and two parallel plate ionization chambers with appropriate radioactive check devices and accompanying inserts. Some tests were performed with photon beams at linear accelerator. Physical visibility check, stability, linearity of response, directional dependence, determination of leakage current, ion recombination and polarity effect and influence of background radiation tests were conducted and analyzed. ResultsThe performance of all Farmer ionization chambers was in agreement to IEC standard as well as for parallel plate type chambers. Long-term stability of older planparallel chamber was somewhat worse than expected. ConclusionRadioactive check devices have proven to be easy to use in clinical environment. According to results of this work Farmer type ionization chambers showed good quality regardless of the time of production, which make them long-term suitable for clinical dosimetry in radiotherapy. Also, for parallel plate type of ionization chambers the results showed fine agreement with the IEC standard, except long-term stability for older chamber, which needs further investigation.

Universal evaluation of MLC models in treatment planning systems based on a common set of dynamic tests

PurposeTo demonstrate the feasibility of characterising MLCs and MLC models implemented in TPSs using a common set of dynamic beams. Materials and methodsA set of tests containing synchronous (SG) and asynchronous sweeping gaps (aSG) was distributed among twenty-five participating centres. Doses were measured with a Farmer-type ion chamber and computed in TPSs, which provided a dosimetric characterisation of the leaf tip, tongue-and-groove, and MLC transmission of each MLC, as well as an assessment of the MLC model in each TPS. Five MLC types and four TPSs were evaluated, covering the most frequent combinations used in radiotherapy departments. ResultsMeasured differences within each MLC type were minimal, while large differences were found between MLC models implemented in clinical TPSs. This resulted in some concerning discrepancies, especially for the HD120 and Agility MLCs, for which differences between measured and calculated doses for some MLC-TPS combinations exceeded 10%. These large differences were particularly evident for small gap sizes (5 and 10 mm), as well as for larger gaps in the presence of tongue-and-groove effects. A much better agreement was found for the Millennium120 and Halcyon MLCs, differences being within ± 5% and ± 2.5%, respectively. ConclusionsThe feasibility of using a common set of tests to assess MLC models in TPSs was demonstrated. Measurements within MLC types were very similar, but TPS dose calculations showed large variations. Standardisation of the MLC configuration in TPSs is necessary. The proposed procedure can be readily applied in radiotherapy departments and can be a valuable tool in IMRT and credentialing audits.

Validation of Monte Carlo simulated absorbed-dose-to-water inside a custom SPECT/CT phantom using active and passive dosimeters: a feasibility study using 99mTc

Objective.This project aims to provide a novel method for performing dosimetry measurements on TRT radionuclides using a custom-made SPECT/CT compatible phantom, common active and passive detectors, and Monte Carlo simulations. In this work we present a feasibility study using 99mTc for a novel approach to obtaining reproducible measurements of absorbed-dose-to-water from radionuclide solutions using active and passive detectors in a custom phantom for the purpose of benchmarking Monte Carlo-based absorbed-dose-to-water estimates. Approach. A cylindrical, acrylic SPECT/CT compatible phantom capable of housing an IBA EFD diode, SNC600c Farmer type ion chamber, and TLD-100 microcubes was designed and built for the purpose of assessing internal absorbed-dose-to-water at various points within a solution of 99mTc. The phantom is equipped with removable inserts that allow for numerous detector configurations and is designed to be used for verification of SPECT/CT-based absorbed-dose estimates with traceable detector measurements at multiple locations. Three experiments were conducted with exposure times ranging from 11 to 21 h with starting activities of approximately 10–16 GBq. Measurement data was compared to Monte Carlo simulations using the egs_chamber user code in EGSnrc 2019. Main results. In general, the ionization chamber measurements agreed with the Monte Carlo simulations within k = 1 uncertainty values (±4% and ±7%, respectively). Measurements from the TLDs yielded results within k = 1 agreement of the MC prediction (±6% and ±5%, respectively). Agreement within k = 1 uncertainty (±6% and ±7%, respectively) was obtained for the diode for one of three conducted experiments. Significance. While relatively large uncertainties remain, the agreement between measured and simulated absorbed-doses provides proof of principal that dosimetry of radionuclide solutions with active detectors may be performed using this type of phantom with potential modifications for beta-emitting radionuclides to be introduced in future work.

Proton beam dosimetry in the presence of magnetic fields using Farmer-type ionization chambers of different radii.

Magnetic resonance-guided proton therapy is promising, as it combines high-contrast imaging of soft tissue with highly conformal dose delivery. However, proton dosimetry in magnetic fields using ionization chambers is challenging since the dose distribution as well as the detector response are perturbed. This work investigates the effect of the magnetic field on the ionization chamber response, and on the polarity and ion recombination correction factors, which are essential for the implementation of a proton beam dosimetry protocol in the presence of magnetic fields. Three Farmer-type cylindrical ionization chambers, the 30013 with 3mm inner radius (PTW, Freiburg, Germany) and two custom built chambers "R1" and "R6" with 1 and 6mm inner radii respectively were placed at the center of an experimental electromagnet (Schwarzbeck Mess - Elektronik, Germany) 2cm depth of an in-house developed 3D printed water phantom. The detector response was measured for a 3×10cm2 field of mono-energetic protons 221.05MeV/u for the three chambers, and with an additional proton beam of 157.43MeV/u for the chamber PTW 30013. The magnetic flux density was varied between 0.1 and 1.0Tesla in steps of 0.1Tesla. At both energies, the ionization chamber PTW 30013 showed a non-linear response as a function of the magnetic field strength, with a decrease of the ionization chamber response of up to 0.27%±0.06% (1 SD) at 0.2Tesla, followed by a smaller effect at higher magnetic field strength. For the chamber R1, the response decreased slightly with the magnetic field strength up to 0.45%±0.12% at 1Tesla, and for the chamber R6, the response decreased up to 0.54%±0.13% at 0.1Tesla, followed by a plateau up to 0.3Tesla, and a weaker effect at higher magnetic field strength. The dependence of the polarity and recombination correction factor on the magnetic field was ⩽0.1% for the chamber PTW 30013. The magnetic field has a small but significant effect on the chamber response in the low magnetic field region for the chamber PTW 30013 and for R6, and in the high magnetic field region for the chamber R1. Corrections may be necessary for ionization chamber measurements, depending on both the chamber volume and the magnetic flux density. No significant effect of the magnetic field on the polarity and recombination correction factor was detected in this work for the ionization chamber PTW 30013.

An effective and optimized patient-specific QA workload reduction for VMAT plans after MLC-modelling optimization

IntroductionMany complexity metrics characterize modulated plans. First, this study aimed at identify the optimal complexity metrics to reduce workload associated to patient-specific quality assurance (PSQA) for our equipment and processes. Second, it intended to optimize our MLC modelling to improve measurement and calculation agreement with expectation of further reducing PSQA workload. MethodsCorrelation and sensitivity at specificity equals to 1 were evaluated for PSQA results and different complexity metrics. Thresholds to stop PSQA were determined. After validation of the optimal complexity metric and threshold for our equipment and process, the MLC modelling was reviewed with a recently published methodology. This method is based on measurements with a Farmer-type ionization chamber of synchronous and asynchronous sweeping gap plans. Effect on the PSQA results and the identified threshold was investigated. ResultsIn our center, the most appropriate complexity metric for reducing our PSQA workload was the Modulation Complexity Score for VMAT (MCSv). The optimization of the MLC modelling significantly reduced the number of controlled plans, specifically for one of our two Varian Clinac. Any plan with a MCSv >= 0.34 is treated without PSQA. ConclusionThis study rationalized and reduced our PSQA workload by approximately 30%. It is a continuing work with new TPS, machine or PSQA equipment. It encourages centers to re-evaluate their MLC modelling as well as assess the benefit of complexity metrics to streamline their PSQA workflow. An easier access, at least for reporting, at best for optimizing plans, into the TPS would be beneficial for the community.

Study of Phantom Length Correction for kV-Cone Beam CT Dose Evaluation Method Using Farmer-type Ionization Chamber and Cylindrical Phantom

In this study, we investigated the necessary phantom length for dose evaluation of kV-CBCT mounted on the linear accelerator using a Farmer-type ionization chamber and cylindrical PMMA phantom, which many radiotherapy facilities own. Furthermore, a phantom length correction factor was proposed to compensate for the lack of scattered radiation contributed from the craniocaudal direction due to the inadequate phantom length. The air-absorbed dose at the center of a cylindrical PMMA phantom measured by a Farmer-type ionization chamber indicated that the contribution of scattered radiation saturates when the phantom length is 300 mm or longer. The phantom length correction factor was calculated from an approximate curve showing the relationship of the charge ratio measured using phantoms of 300 mm and 150 mm in length to the X-ray beam width. The air-absorbed dose measured with the 150-mm length phantom, corrected by the phantom length correction factor, showed a 1.61% dose difference from the air-absorbed dose measured with the 300-mm length phantom. In this study, the air-absorbed dose at the center of the phantom could be estimated over a wide X-ray beam width only using a 150-mm length cylindrical PMMA phantom. The method proposed in this study could be used in any radiation therapy facility.

Characteristics of photopolymerized tissue equivalent plastic scintillator in high dose rate radiotherapy”

Recently, as the biological effects of ultrahigh-dose-rate radiation therapy (flash RT) have been verified, many kinds of research on the clinical application of high-dose-rate radiation therapy have been performed. Moreover, various studies are being performed to determine the absorbed dose in the volume of cancers. In this study, to evaluate the absorbed dose of the human body during high-dose-rate radiation therapy, a dosimeter system was developed using a tissue equivalent plastic scintillator having the same geometric structure as the Farmer-type ionization chamber, and its characteristics were evaluated. The plastic scintillator was based on PMMA, and PPO was used as the primary scintillator and POPOP as the wavelength shifter. An Irgacure 819 photo-initiator and a Hg lamp were used to polymerize of PMMA plastic scintillator. The fabricated plastic scintillator was characterized for conventional LINAC X-ray and 45 MeV proton beams. The results of this study combined with 3D printing technology are planned to be used as dosimeters for volumetric dosimetry of normal and cancerous tissues during radiation therapy.

Use of a PLA sleeve to remove electron enhancement in superficial X-ray therapy.

A newly installed superficial X-ray unit was found to produce enhanced electron dose at the skin surface. The ACPSEM kilovoltage dosimetry recommendations suggest using nail varnish within the treatment cones as a method to reduce this dose. In this study, a 3D PLA sleeve was produced and used as an alternative to the nail varnish for energies between 55 and 100kV. Further, plastic wrap was also investigated as an alternative method to reduce dose. It was found that a 1mm printed sleeve, inserted into the treatment cone sufficiently reduced the enhanced dose as measured with a thin-window Exradin chamber to within 3.3% of the dose measured with a Farmer-type ionisation chamber. The use of plastic wrap also reduced the enhanced dose, but impracticalities in its use make it non-viable for routine clinical use.

Comparison between the response in air and in water of two ionization chambers exposed to low-energy X-rays

This work investigated the effect of two different media in the response of two Farmer-type ionization chambers exposed to low-energy x-rays from 27 kV – 155 kV. The measurements were performed in air and in liquid water. Independent of the ionization chamber, at effective energies below 30 keV (80 kV), the charge collected in liquid water can be up to 84% less than that generated in air. The lower is the energy, the smaller is the response in water compared to air. This can be associated to the rapid attenuation of the low-energy photon fluence by the water depth. At effective energies greater than 30 keV, the response in water becomes larger than that in air, reaching a maximum of 27% and 35% at 65 keV (150 kV) for A19 and A12 chamber, respectively. The difference in response between the two media is consistently greater for ionization chamber A19 at energy above 25 keV.

Nontarget and Out-of-Field Doses from Electron Beam Radiotherapy.

In clinical radiotherapy, the most important aspects are the dose distribution in the target volume and healthy organs, including out-of-field doses in the body. Compared to photon beam radiation, dose distribution in electron beam radiotherapy has received much less attention, mainly due to the limited range of electrons in tissues. However, given the growing use of electron intraoperative radiotherapy and FLASH, further study is needed. Therefore, in this study, we determined out-of-field doses from an electron beam in a phantom model using two dosimetric detectors (diode E and cylindrical Farmer-type ionizing chamber) for electron energies of 6 MeV, 9 MeV and 12 MeV. We found a clear decrease in out-of-field doses as the distance from the field edge and depth increased. The out-of-field doses measured with the diode E were lower than those measured with the Farmer-type ionization chamber at each depth and for each electron energy level. The out-of-field doses increased when higher energy megavoltage electron beams were used (except for 9 MeV). The out-of-field doses at shallow depths (1 or 2 cm) declined rapidly up to a distance of 3 cm from the field edge. This study provides valuable data on the deposition of radiation energy from electron beams outside the irradiation field.

Direct determination of k Q for Farmer-type ionization chambers in a clinical scanned carbon-ion beam using water calorimetry

Within two studies, k Q factors for two Farmer-type ionization chambers have been experimentally determined by means of water calorimetry in the entrance channel (EC) of a monoenergetic carbon-ion beam (Osinga-Blättermann et al 2017 Phys. Med. Biol. 62 2033–54) and for a passively modulated spread-out Bragg peak (SOBP) (Holm et al 2021 Phys. Med. Biol. 66 145012). Both studies were performed at the Heidelberg Ion Beam Therapy Center (HIT) using the PTB portable water calorimeter but applying different initial beam energies of 429 MeV u−1 for the EC and 278 MeV u−1 for the SOBP as well as different scanning patterns of the irradiated field. Comparing their results revealed differences between the experimental k Q factors of up to 1.9% between the EC and the SOBP. To further investigate this unexpected difference, we performed additional k Q determinations for the EC of an 278 MeV u−1 monoenergetic carbon-ion beam and reevaluated the original data of Osinga-Blättermann et al (2017 Phys. Med. Biol. 62 2033–54). This new experimental data indicated no difference between the k Q factors for the EC and the SOBP and the reevaluation led to a substantial reduction of the originally published k Q factors for the EC of the 429 MeV u−1 beam (Osinga-Blättermann et al 2017 Phys. Med. Biol. 62 2033–54). Finally, no significant difference between the data for the EC and the data for the SOBP can be found within the standard measurement uncertainty of experimental k Q factors of 0.8%. The results presented here are intended to correct and replace the k Q data published by Osinga-Blättermann et al (2017 Phys. Med. Biol. 62 2033–54) and in Osinga-Blättermann and Krauss (2018 Phys. Med. Biol. 64 015009).

Technical note: The impact of storage humidity on the response of reference-class ionization chambers.

A number of Farmer-type ionization chambers were tested against storage humidity to confirm whether they satisfied the criteria for the long-term stability of reference-class ionizationchambers. The ionization chambers were stored for several months in an environment with relative humidity adjustable from 20% to 80%. The ionization chambers were removed from the storage environment at variable intervals ranging from 1 to 70days and irradiated in a Co-60 radiation beam. The responses for each ionization chamber were evaluated from the measured currents corrected for the atmospheric air density, and were compared with those predicted by the Co-60 half-life. Certain ionization chambers gave a constant relative response regardless of the storage humidity, while the relative responses of two types of ionization chambers changed as a function of the storage humidity. The difference between the relative responses for the low (20-30%) and high (70-80%) storage relative humidity was 0.7%. The response was larger for the high relative humidity storage. Immediately after the storage humidity changed, the relative response started to change by the day, and it took approximately 2 weeks to 2 months for the relative response to converge. For one type of the ionization chamber, the plastic outer wall and the outer electrode were replaced with those made of solid graphite, and it was confirmed that the remodeled ionization chamber did not exhibit the responsechange. The present results and previous reports by other authors indicate that the magnitude of the change depends on the magnitude of the water absorption of the plastic used for the outer wall and/or the electrode of the ionization chamber. Thus, it is important in the selection of the reference-class ionization chamber to note the material and structure of the outer wall and electrode of the ionization chamber. If the ionization chamber has a hygroscopic wall and electrode and it is used as a reference ionization chamber, it is necessary to pay additional attention to the humidity difference for the storage, daily irradiation, and yearly calibration especially in regions with large seasonal humidityfluctuations.

The Design of an Audit Test for 60Co Brachytherapy Treatment Planning System.

Background:Auditing the treatment planning system (TPS) software for a radiotherapy unit is of paramount importance in any radiation therapy department. A Plexiglas phantom was proposed to measure the ionization of 60Co high dose rate (HDR) source and compare dose points in the planning system for auditing and verifying TPS.Methods:Auditing was performed using a Plexiglas phantom in an end-to-end test, and relative dose points were detected by a farmer-type ionization chamber and compared with the relative dose of similar points in TPS. The audit results were determined as pass optimal level (<3.3%), pass action level (between 3.3% and 5%), and out of tolerance (>5%).Results:The comparison of the collected data revealed that 80% of the measured values were ≤5% in the pass level, and 20% of the points were out of tolerance (between 5% and 6.99%).Conclusion:This study documented the appropriateness of the dosimetry audit test and this phantom design for the HDR brachytherapy TPS.

Development of dosimetric procedures for experimental ultra-high dose rate irradiation at a clinical linear accelerator

As radiotherapy using ultra-high dose rates has gained new interest, the dosimetric challenges arising at these conditions needs to be addressed. Ionization chambers suffer from a large decrease in ion collection efficiency due to ion recombination, making on-line dosimetry difficult. In this work we present experimental setups and dosimetric procedures for FLASH irradiation of cells, zebrafish embryos and small animals using a 10 MeV electron beam at a modified clinical linear accelerator, and describe the dosimetric steps required to initiate clinical trials. The dosimetric equipment used for our pre-clinical experiments consisted of radiochromic film, thermoluminescent dosimeters, a Farmer-type ionization chamber and phantom material mimicking the experimental setup for irradiation. In preparation for small animal irradiation, dose profiles and depth dose curves were measured for all collimator sizes. The average dose rates were ≥620 Gy/s, ≥640 Gy/s and ≥400 Gy/s for cells, zebrafish embryos and small animals, respectively.

Calibration of a Farmer Type Ionization Chamber in a Therapeutic Proton Scanning Beam Using a Faraday Cup and the Geant4 Monte Carlo Program

Calibration of a Farmer Type Ionization Chamber in a Therapeutic Proton Scanning Beam Using a Faraday Cup and the Geant4 Monte Carlo Program

Quantification of the temperature equilibrium time of the cavity in parallel-plate-type ionization chambers by thermal analysis.

Temperature corrections are necessary to account for the varying mass of air in the cavity volume of a vented ionization chamber. The temporal response resulting from temperature changes in a cylindrical and/or Farmer-type ionization chamber, which is the standard dosimeter, has been thoroughly discussed by some researchers. The purpose of this study was to characterise and analyse the dependence of the cavity air temperature of the parallel-plate-type ionization chamber on changes in the ambient temperature. Ionization chambers NACP-02 (IBA Dosimetry, GmbH) and Advanced Markus TN34045 (PTW, Freiburg) were modelled using thermal analysis software to present the temperature equilibrium time and the entire ionization chamber temperature distribution. The temporal response of each ionization chamber was measured for comparing the calculation results of the thermal analysis. The ionization chamber cavities of NACP-02 and TN34045 reached complete equilibrium in 670 and 750 s, respectively. Heat transfer occurred faster at the centre of the front wall of TN34045 than at the outside of the centre except for the edges. Further, the non-uniformity of temperature in the cavity was in the range of 24.2–24.8°C for NACP-02 and 23.7–24.4°C for TN34045 at 200 s after the ionization chamber was installed in the water phantom. The previous proposal to wait for about 15 mins after submerging the chamber in a water phantom before the measurement is demonstrated to be appropriate for parallel-plate-type ionization chambers.

Traceable reference dosimetry in MRI guided radiotherapy using alanine: calibration and magnetic field correction factors of ionisation chambers

Magnetic resonance imaging (MRI)-guided radiotherapy (RT) (MRIgRT) falls outside the scope of existing high energy photon therapy dosimetry protocols, because those protocols do not consider the effects of the magnetic field on detector response and on absorbed dose to water. The aim of this study is to evaluate and demonstrate the traceable measurement of absorbed dose in MRIgRT systems using alanine, made possible by the characterisation of alanine sensitivity to magnetic fields reported previously by Billas et al (2020 Phys. Med. Biol. 65 115001), in a way which is compatible with existing standards and calibrations available for conventional RT. In this study, alanine is used to transfer absorbed dose to water to MRIgRT systems from a conventional linac. This offers an alternative route for the traceable measurement of absorbed dose to water, one which is independent of the transfer using ionisation chambers. The alanine dosimetry is analysed in combination with measurements with several Farmer-type chambers, PTW 30013 and IBA FC65-G, at six different centres and two different MRIgRT systems (Elekta Unity™ and ViewRay MRIdian™). The results are analysed in terms of the magnetic field correction factors, and in terms of the absorbed dose calibration coefficients for the chambers, determined at each centre. This approach to reference dosimetry in MRIgRT produces good consistency in the results, across the centres visited, at the level of 0.4% (standard deviation). Farmer-type ionisation chamber magnetic field correction factors were determined directly, by comparing calibrations in some MRIgRT systems with and without the magnetic field ramped up, and indirectly, by comparing calibrations in all the MRIgRT systems with calibrations in a conventional linac. Calibration coefficients in the MRIgRT systems were obtained with a standard uncertainty of 1.1% (Elekta Unity™) and 0.9% (ViewRay MRIdian™), for three different chamber orientations with respect to the magnetic field. The values obtained for the magnetic field correction factor in this investigation are consistent with those presented in the summary by de Pooter et al (2021 Phys. Med. Biol. 66 05TR02), and would tend to support the adoption of a magnetic field correction factor which depends on the chamber type, PTW 30013 or IBA FC65-G.