Experimental characterization of four ionization chamber types in magnetic fields including intra-type variation.

Background and purpose: For dosimetry in magnetic resonance (MR) guided radiotherapy, assessing the magnetic field correction factors of air-vented ionization chambers is crucial. Novel MR-optimized chambers reduce MR-imaging artefacts, enhancing their quality assurance utility. This study aimed to characterize two new MR-optimized ionization chambers with sensitive volumes of 0.07 and 0.016 cm3 regarding magnetic field correction factors and intra-type variation and compare them to their conventional counterparts. Material and methods: Five chambers of each type were evaluated in a water phantom, using a clinical linear accelerator and an electromagnet, as well as a 1.5 T MR-linac system. The magnetic field correction factor kB→,Q, addressing the change of response caused by a magnetic field, was assessed together with its intra-type variation. MR-optimized and conventional chambers were compared using a Mann-Whitney U-Test. Results: Considering 1.5 T and a perpendicular chamber orientation, we observed significant differences in the magnetic field-induced change in chamber reading between the two 0.016 cm3 chamber versions (p = 0.03). For a 7 MV beam, MR-optimized chambers (0.016/0.07 cm3) showed kB→,Q values of 1.0426(66) and 1.0463(44), compared to 1.0319(53) and 1.0480(41) of their conventional counterparts. In anti-parallel orientation, kB→,Q was 1.0012(69) and 0.9863(49) for the MR-optimized chambers. The average intra-type variation of kB→,Q over all chamber types was 0.3%. Conclusion: Magnetic field correction factors were successfully determined for four ionization chamber types, including two new MR-optimized versions, allowing their use in MR-linac absolute dosimetry. Evaluation of the intra-type variation enabled the assessment of their contribution to the uncertainty of tabulated kB→,Q.

Charge collection efficiency of commercially available parallel-plate ionisation chambers in ultra-high dose-per-pulse electron beams.

Objective. This investigation aims to experimentally determine the charge collection efficiency (CCE) of six commercially available parallel-plate ionisation chamber (PPIC) models in ultra-high dose-per-pulse (UHDPP) electron beams.Approach. The CCE of 22 PPICs has been measured in UHDPP electron beams at the National Metrology Institution of Germany (PTB). The CCE was determined for a dose per pulse (DPP) range between 0.1 and 6.4 Gy (pulse duration of 2.5µs). The results obtained with the different PPICs were compared to evaluate the reproducibility, intra- and inter-model variation, and the performance of a CCE empirical model.Main results. The intra-model variation was, on average, 4.0%, which is more than three times the total combined relative standard uncertainty and was found to be greater at higher DPP (up to 20%). The inter-model variation for the PPIC with 2 mm electrode spacing, which was found to be, on average, 10%, was also significant compared to the relative uncertainty and the intra-model variation. The observed CCE variation could not be explained only by the expected deviation of the electrode spacing from the nominal value within the manufacturing tolerance. It should also be noted that a substantial polarity effect, between 0.914(5) and 1.201(3), was observed, and significant intra- and inter-model variation was observed on this effect.Significance. For research and pre-clinical study, the commercially available PPIC with a well-known CCE (directly measured for the specific chamber) and with a small electrode spacing could be used for relative and absolute dosimetry with a lower-limit uncertainty of 1.6% (k= 1) in the best case. However, to use a PPIC as a secondary standard in UHDPP electron beams for clinical purposes would require new model development to reduce the ion recombination, the polarity effect, and the total standard uncertainty on the dose measurement.

The PTB water calorimeter for determining the absolute absorbed dose to water in ultra-high pulse dose rate electron beams.

Objective. The purpose of this investigation is to establish the water calorimeter as the primary standard in PTB's ultra-high pulse dose rate (UHPDR) 20 MeV reference electron beams.Approach. The calorimetric measurements were performed at the PTB research linac facility using the UHPDR reference electron beam setups that enable a dose per pulse between about 0.1 Gy and 6 Gy. The beam is monitored by an in-flange integrating current transformer. The correction factors required to determine the absorbed dose to water were evaluated using thermal and Monte Carlo simulations. Measurements were performed with different total doses delivered per pulse by modifying the instantaneous dose rate within a pulse and by changing the pulse length. The obtained temperature-time traces were compared to the simulated ones to validate the thermal simulations. In addition, absorbed-dose-to-water measurements obtained using the secondary standard alanine dosimeter system were compared to measurements performed with the primary standard.Main results. The simulated and measured temperature-time traces were shown to be consistent, within combined uncertainties, with one another. Measurements with alanine dosimeters proved to be consistent withink= 1 of the total combined uncertainty with the absorbed dose to water determined using the primary standard.Significance. The total relative standard uncertainty of absorbed dose to water determined using the PTB water calorimeter primary standard in UHPDR electron beams was estimated to be less than 0.5%, and the combined correction factors were found to deviate from 1 by less than 1% for both PTB UHPDR 20 MeV reference electron beams. The water calorimeter is therefore considered to be an established primary standard for the higher energy UHPDR reference electron beams.

Experimental and Monte Carlo-based determination of magnetic field correction factors k B , Q $k_{B,Q}$ in high-energy photon fields for two ionization chambers.

BACKGROUND: The integration of magnetic resonance tomography into clinical linear accelerators provides high-contrast, real-time imaging during treatment and facilitates online-adaptive workflows in radiation therapy treatments. The associated magnetic field also bends the trajectories of charged particles via the Lorentz force, which may alter the dose distribution in a patient or a phantom and affects the dose response of dosimetry detectors. PURPOSE: To perform an experimental and Monte Carlo-based determination of correction factors k B , Q $k_{B,Q}$ , which correct the response of ion chambers in the presence of external magnetic fields in high-energy photon fields. METHODS: The response variation of two different types of ion chambers (Sun Nuclear SNC125c and SNC600c) in strong external magnetic fields was investigated experimentally and by Monte Carlo simulations. The experimental data were acquired at the German National Metrology Institute, PTB, using a clinical linear accelerator with a nominal photon energy of 6 MV and an external electromagnet capable of generating magnetic flux densities of up to 1.5 T in opposite directions. The Monte Carlo simulation geometries corresponded to the experimental setup and additionally to the reference conditions of IAEA TRS-398. For the latter, the Monte Carlo simulations were performed with two different photon spectra: the 6 MV spectrum of the linear accelerator used for the experimental data acquisition and a 7 MV spectrum of a commercial MRI-linear accelerator. In each simulation geometry, three different orientations of the external magnetic field, the beam direction and the chamber orientation were investigated. RESULTS: Good agreement was achieved between Monte Carlo simulations and measurements with the SNC125c and SNC600c ionization chambers, with a mean deviation of 0.3% and 0.6%, respectively. The magnitude of the correction factor k B , Q $k_{B,Q}$ strongly depends on the chamber volume and on the orientation of the chamber axis relative to the external magnetic field and the beam directions. It is greater for the SNC600c chamber with a volume of 0.6 cm3 than for the SNC125c chamber with a volume of 0.1 cm3 . When the magnetic field direction and the chamber axis coincide, and they are perpendicular to the beam direction, the ion chambers exhibit a calculated overresponse of less than 0.7(6)% (SNC600c) and 0.3(4)% (SNC125c) at 1.5 T and less than 0.3(0)% (SNC600c) and 0.1(3)% (SNC125c) for 0.35 T for nominal beam energies of 6 MV and 7 MV. This chamber orientation should be preferred, as k B , Q $k_{B,Q}$ may increase significantly in other chamber orientations. Due to the special geometry of the guard ring, no dead-volume effects have been observed in any orientation studied. The results show an intra-type variation of 0.17% and 0.07% standard uncertainty (k=1) for the SNC125c and SNC600c, respectively. CONCLUSION: Magnetic field correction factors k B , Q $k_{B,Q}$ for two different ion chambers and for typical clinical photon beam qualities were presented and compared with the few data existing in the literature. The correction factors may be applied in clinical reference dosimetry for existing MRI-linear accelerators.

Absorbed-dose-to-water measurement using alanine in ultra-high-pulse-dose-rate electron beams.

Objective. The aim of the presented study is to evaluate the dose response of the PTB's secondary standard system, which is based on alanine and electron spin resonance (ESR) spectroscopy measurement, in ultra-high-pulse-dose-rate (UHPDR) electron beams.Approach. The alanine dosimeter system was evaluated in the PTB's UHPDR electron beams (20 MeV) in a range of 0.15-6.2 Gy per pulse. The relationship between the obtained absorbed dose to water per pulse and the in-beamline charge measurement of the electron pulses acquired using an integrating current transformer (ICT) was evaluated. Monte Carlo simulations were used to determine the beam quality conversion and correction factors required to perform alanine dosimetry.Main results. The beam quality conversion factor from the reference quality60Co to 20 MeV obtained by Monte Carlo simulation, 1.010(1), was found to be within the standard uncertainty of the consensus value, 1.014(5). The dose-to-water relative standard uncertainty was determined to be 0.68% in PTB's UHPDR electron beams.Significance. In this investigation, the dose-response of the PTB's alanine dosimeter system was evaluated in a range of dose per pulse between 0.15 Gy and 6.2 Gy and no evidence of dose-response dependency of the PTB's secondary standard system based on alanine was observed. The alanine/ESR system was shown to be a precise dosimetry system for evaluating absorbed dose to water in UHPDR electron beams.

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

PURPOSE: 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. METHODS: In this paper, the calorimeter system is used to investigate the potential influence of dose per pulses delivered up to 5.6 Gy, 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. RESULTS: 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 0.5 mm 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 et al. https://doi.org/10.1088/0026-1394/52/2/272) but has the advantage of being a real-time detector. CONCLUSION: 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.

Characterization of the PTB ultra-high pulse dose rate reference electron beam.

Purpose. This investigation aims to present the characterisation and optimisation of an ultra-high pulse dose rate (UHPDR) electron beam at the PTB facility in Germany. A Monte Carlo beam model has been developed for dosimetry study for future investigation in FLASH radiotherapy and will be presented.Material and methods. The 20 MeV electron beams generated by the research linear accelerator has been characterised both in-beamline with profile monitors and magnet spectrometer, and in-water with a diamond detector prototype. The Monte Carlo model has been used to investigate six different setups to enable different dose per pulse (DPP) ranges and beam sizes in water. The properties of the electron radiation field in water have also been characterised in terms of beam size, quality specifierR50and flatness. The beam stability has also been studied.Results. The difference between the Monte-Carlo simulated and measuredR50was smaller than 0.5 mm. The simulated beam sizes agreed with the measured ones within 2 mm. Two suitable setups have been identified for delivering reference UHPDR electron beams. The first one is characterised by a SSD of 70 cm, while in the second one an SSD of 90 cm is used in combination with a 2 mm aluminium scattering plates. The two set-ups are quick and simple to install and enable an expected overall DPP range from 0.13 Gy up to 6.7 Gy per pulse.Conclusion. The electron beams generated by the PTB research accelerator have shown to be stable throughout the four-months length of this investigation. The Monte Carlo models have shown to be in good agreement for beam size and depth dose and within 1% for the beam flatness. The diamond detector prototype has shown to be a promising tool to be used for relative measurements in UHPDR electron beams.

Cema-based formalism for the determination of absorbed dose for high-energy photon beams.

PURPOSE: Determination of absorbed dose is well established in many dosimetry protocols and considered to be highly reliable using ionization chambers under reference conditions. If dosimetry is performed under other conditions or using other detectors, however, open questions still remain. Such questions frequently refer to appropriate correction factors. A converted energy per mass (cema)-based approach to formulate such correction factors offers a good understanding of the specific response of a detector for dosimetry under various measuring conditions and thus an estimate of pros and cons of its application. METHODS: Determination of absorbed dose requires the knowledge of the beam quality correction factor kQ,Qo , where Q denotes the quality of a user beam and Qo is the quality of the radiation used for calibration. In modern Monte Carlo (MC)-based methods, kQ,Qo is directly derived from the MC-calculated dose conversion factor, which is the ratio between the absorbed dose at a point of interest in water and the mean absorbed dose in the sensitive volume of an ion chamber. In this work, absorbed dose is approximated by the fundamental quantity cema. This approximation allows the dose conversion factor to be substituted by the cema conversion factor. Subsequently, this factor is decomposed into a product of cema ratios. They are identified as the stopping power ratio water to the material in the sensitive detector volume, and as the correction factor for the fluence perturbation of the secondary charged particles in the detector cavity caused by the presence of the detector. This correction factor is further decomposed with respect to the perturbation caused by the detector cavity and that caused by external detector properties. The cema-based formalism was subsequently tested by MC calculations of the spectral fluence of the secondary charged particles (electrons and positrons) under various conditions. RESULTS: MC calculations demonstrate that considerable fluence perturbation may occur particularly under non-reference conditions. Cema-based correction factors to be applied in a 6-MV beam were obtained for a number of ionization chambers and for three solid-state detectors. Feasibility was shown at field sizes of 4 × 4 and 0.5 cm × 0.5 cm. Values of the cema ratios resulting from the decomposition of the dose conversion factor can be well correlated with detector response. Under the small field conditions, the internal fluence correction factor of ionization chambers is considerably dependent on volume averaging and thus on the shape and size of the cavity volume. CONCLUSIONS: The cema approach is particularly useful at non-reference conditions including when solid-state detectors are used. Perturbation correction factors can be expressed and evaluated by cema ratios in a comprehensive manner. The cema approach can serve to understand the specific response of a detector for dosimetry to be dependent on (a) radiation quality, (b) detector properties, and (c) electron fluence changes caused by the detector. This understanding may also help to decide which detector is best suited for a specific measurement situation.

The dose response of PTW microDiamond and microSilicon in transverse magnetic field under small field conditions.

The aim of the present work is to investigate the behavior of two diode-type detectors (PTW microDiamond 60019 and PTW microSilicon 60023) in transverse magnetic field under small field conditions. A formalism based on TRS 483 has been proposed serving as the framework for the application of these high-resolution detectors under these conditions. Measurements were performed at the National Metrology Institute of Germany (PTB, Braunschweig) using a research clinical linear accelerator facility. Quadratic fields corresponding to equivalent square field sizesSbetween 0.63 and 4.27 cm at the depth of measurement were used. The magnetic field strength was varied up to 1.4 T. Experimental results have been complemented with Monte Carlo simulations up to 1.5 T. Detailed simulations were performed to quantify the small field perturbation effects and the influence of detector components on the dose response. The does response of both detectors decreases by up to 10% at 1.5 T in the largest field size investigated. InS = 0.63 cm, this reduction at 1.5 T is only about half of that observed in field sizesS > 2 cm for both detectors. The results of the Monte Carlo simulations show agreement better than 1% for all investigated conditions. Due to normalization at the machine specific reference field, the resulting small field output correction factors for both detectors in magnetic fieldkQclin,QmsrBare smaller than those in the magnetic field-free case, where correction up to 6.2% at 1.5 T is required for the microSilicon in the smallest field size investigated. The volume-averaging effect of both detectors was shown to be nearly independent of the magnetic field. The influence of the enhanced-density components within the detectors has been identified as the major contributors to their behaviors in magnetic field. Nevertheless, the effect becomes weaker with decreasing field size that may be partially attributed to the deficiency of low energy secondary electrons originated from distant locations in small fields.

The role of the construction and sensitive volume of compact ionization chambers on the magnetic field-dependent dose response.

PURPOSE: The magnetic-field correction factors k B , Q of compact air-filled ionization chambers have been investigated experimentally and using Monte Carlo simulations up to 1.5 T. The role of the nonsensitive region within the air cavity and influence of the chamber construction on its dose response have been elucidated. MATERIALS AND METHODS: The PTW Semiflex 3D 31021, PinPoint 3D 31022, and Sun Nuclear Cooperation SNC125c chambers were studied. The k B , Q factors were measured at the experimental facility of the German National Metrology Institute (PTB) up to 1.4 T using a 6 MV photon beam. The chambers were positioned with the chamber axis perpendicular to the beam axis (radial); and parallel to the beam axis (axial). In both cases, the magnetic field was directed perpendicular to both the beam axis and chamber axis. Additionally, the sensitive volumes of these chambers have been experimentally determined using a focused proton microbeam and finite element method. Beside the simulations of k B , Q factors, detailed Monte Carlo technique has been applied to analyse the secondary electron fluence within the air cavity, that is, the number of secondary electrons and the average path length as a function of the magnetic field strength. RESULTS: A nonsensitive volume within the air cavity adjacent to the chamber stem for the PTW chambers has been identified from the microbeam measurements and FEM calculations. The dose response of the three investigated ionization chambers does not deviate by more than 4% from the field-free case within the range of magnetic fields studied in this work for both the radial and axial orientations. The simulated k B , Q for the fully guarded PTW chambers deviate by up to 6% if their sensitive volumes are not correctly considered during the simulations. After the implementation of the sensitive volume derived from the microbeam measurements, an agreement of better than 1% between the experimental and Monte Carlo k B , Q factors for all three chambers can be achieved. Detailed analysis reveals that the stem of the PTW chambers could give rise to a shielding effect reducing the number of secondary electrons entering the air cavity in the presence of magnetic field. However, the magnetic field dependence of their path length within the air cavity is shown to be weaker than for the SNC125c chamber, where the length of the air cavity is larger than its diameter. For this chamber it is shown that the number of electrons and their path lengths in the cavity depend stronger on the magnetic field. DISCUSSION AND CONCLUSION: For clinical measurements up to 1.5 T, the required k B , Q corrections of the three chambers could be kept within 3% in both the investigated chamber orientations. The results reiterate the importance of considering the sensitive volume of fully guarded chambers, even for the investigated compact chambers, in the Monte Carlo simulations of chamber response in magnetic field. The resulting magnetic field-dependent dose response has been demonstrated to depend on the chamber construction, such as the ratio between length and the diameter of the air cavity as well as the design of the chamber stem.

Reference dosimetry in MRI-linacs: evaluation of available protocols and data to establish a Code of Practice.

With the rapid increase in clinical treatments with MRI-linacs, a consistent, harmonized and sustainable ground for reference dosimetry in MRI-linacs is needed. Specific for reference dosimetry in MRI-linacs is the presence of a strong magnetic field. Therefore, existing Code of Practices (CoPs) are inadequate. In recent years, a vast amount of papers have been published in relation to this topic. The purpose of this review paper is twofold: to give an overview and evaluate the existing literature for reference dosimetry in MRI-linacs and to discuss whether the literature and datasets are adequate and complete to serve as a basis for the development of a new or to extend existing CoPs. This review is prefaced with an overview of existing MRI-linac facilities. Then an introduction on the physics of radiation transport in magnetic fields is given. The main part of the review is devoted to the evaluation of the literature with respect to the following subjects: • beam characteristics of MRI-linac facilities; • formalisms for reference dosimetry in MRI-linacs; • characteristics of ionization chambers in the presence of magnetic fields; • ionization chamber beam quality correction factors; and • ionization chamber magnetic field correction factors. The review is completed with a discussion as to whether the existing literature is adequate to serve as basis for a CoP. In addition, it highlights subjects for future research on this topic.

The European Joint Research Project UHDpulse - Metrology for advanced radiotherapy using particle beams with ultra-high pulse dose rates.

UHDpulse - Metrology for advanced radiotherapy using particle beams with ultra-high pulse dose rates is a recently started European Joint Research Project with the aim to develop and improve dosimetry standards for FLASH radiotherapy, very high energy electron (VHEE) radiotherapy and laser-driven medical accelerators. This paper gives a short overview about the current state of developments of radiotherapy with FLASH electrons and protons, very high energy electrons as well as laser-driven particles and the related challenges in dosimetry due to the ultra-high dose rate during the short radiation pulses. We summarize the objectives and plans of the UHDpulse project and present the 16 participating partners.

Experimental determination of magnetic field correction factors for ionization chambers in parallel and perpendicular orientations.

Magnetic field correction factors are needed for absolute dosimetry in magnetic resonance (MR)-linacs. Currently experimental data for magnetic field correction factors, especially for small volume ionization chambers, are largely lacking. The purpose of this work is to establish, independent methods for the experimental determination of magnetic field correction factors [Formula: see text] in an orientation in which the ionization chamber is parallel to the magnetic field. The aim is to confirm previous experiments on the determination of Farmer type ionization chamber correction factors and to gather information about the usability of small-volume ionization chambers for absolute dosimetry in MR-linacs. The first approach to determine [Formula: see text] is based on a cross-calibration of measurements using a conventional linac with an electromagnet and an MR-linac. The absolute influence of the magnetic field in perpendicular orientation is quantified with the help of the conventional linac and the electromagnet. The correction factors for the parallel orientation are then derived by combining these measurements with relative measurements in the MR-linac. The second technique utilizes alanine electron paramagnetic resonance dosimetry. The alanine system as well as several ionization chambers were directly calibrated with the German primary standard for absorbed dose to water. Magnetic field correction factors for the ionization chambers were determined by a cross-calibration with the alanine in an MR-linac. Important quantities like [Formula: see text] for Farmer type ionization chambers in parallel orientation and the change of the dose to water due the magnetic field [Formula: see text] have been confirmed. In addition, magnetic field correction factors have been determined for small volume ionization chambers in parallel orientation. The electromagnet-based measurements of [Formula: see text] for [Formula: see text] MR-linacs and parallel ionization chamber orientations resulted in 0.9926(22), 0.9935(31) and 0.9841(27) for the PTW 30013, the PTW 31010 and the PTW 31021, respectively. The measurements based on the second technique resulted in values for [Formula: see text] of 0.9901(72), 0.9955(72), and 0.9885(71). Both methods show excellent accuracy and reproducibility and are therefore suitable for the determination of magnetic field correction factors. Small-volume ionization chambers showed a variation in the resulting values for [Formula: see text] and should be cross-calibrated instead of using tabulated values for correction factors.

The dose response of high-resolution diode-type detectors and the role of their structural components in strong magnetic field.

PURPOSE: This study aims to investigate the dose response of diode-type detectors in the presence of strong magnetic field and to understand the underlying mechanisms leading to the observed magnetic field dependence by close examinations on the role of the detector's design. MATERIALS AND METHODS: Three clinical diode-type detectors (PTW microSilicon type 60023, PTW microDiamond type 60019, and IBA Razor diode) have been studied. Measurements were performed at the linear accelerator experimental facility of the German National Metrology Institute (PTB, Braunschweig) with electromagnets up to 1.4 T to obtain the magnetic field correction factors k B , Q . The experimental results were compared to Monte Carlo simulations. Stepwise modifications of the detectors' models were performed to characterize the contributions of the structural components toward the magnetic field-dependent dose response. Additionally, systematic Monte Carlo study was conducted to elucidate the influence of the structural layers with varying density located above and beneath the detector's sensitive volume. RESULTS: The dose response of all investigated detectors decreases with magnetic field. As a result, the associated k B , Q factors increase by approximately 10% for the PTW detectors, and by 5% for the IBA Razor diode at 1.5 T. The sensitive volume itself was shown to cause negligible effect but the diode substrate with enhanced density situated directly below the sensitive volume contributes strongest to the observed magnetic field dependence. Systematic simulations revealed that k B , Q increases with magnetic field if the density of the structural layer located beneath the sensitive volume is higher than that of normal water (>1 g/cm3 ). In the case where the layer consists of low-density water (1.2 mg/cm3 ), k B , Q decreases with the magnetic field strength. On the contrary , if the structural layer with varying density is situated above the sensitive volume, the reversed effect could be observed. DISCUSSION AND CONCLUSION: The experimental and Monte Carlo results demonstrated that the dose response of the investigated diode-type detectors decreases in magnetic field. This observation can be generally attributed to the common construction of diode-type detectors, where structural components with enhanced density, for example the diode substrate, are situated below the sensitive volume. The results provide deeper insights into the behavior of clinical diode detectors when used in strong magnetic field.

Chemical radiation dosimetry in magnetic fields: characterization of a Fricke-type chemical detector in 6 MV photon beams and magnetic fields up to 1.42 T.

In magnetic resonance guided radiotherapy (MRgRT) radiation dose measurements needs to be performed in the presence of a magnetic field. In this study, the influence of magnetic fields on the readings of a Fricke detector, a chemical dosimeter, have been investigated in 6 MV photon beams. This type of detector has been chosen, as the Federal Office of Metrology (METAS, Switzerland) has great experience with Fricke dosimetry and since it is not expected that this detector is greatly affected by the presence of a magnetic field. Magnetic fields with field strengths between 0 T and 1.42 T were applied during the detector irradiation. In a 5 × 10 cm2 irradiation field, the Fricke readings are affected less than 0.9% by the applied magnetic fields. Taking the altered dose distribution due to the magnetic field ([Formula: see text]) into account, the magnetic field correction factors ([Formula: see text]) for the Fricke detector at 0.35 T and 1.42 T are determined to be 0.9948 and 0.9980, respectively. These small corrections hardly exceed the measurement uncertainties. Hence, we could proof that the Fricke detector is not significantly influenced by the presence of a magnetic field. The Fricke detector was also tested for the feasibility of measuring output factors in the presence of magnetic fields. For irradiation field sizes larger than the detector (>2 × 2 cm2), comparable results were obtained as for other detectors. The output factors decrease when a magnetic field is applied. This effect is more pronounce for larger magnetic field strengths and smaller irradiation fields due to shifts of the depth dose curves and asymmetry of lateral dose profiles.

Method to quickly and accurately calculate absorbed dose from therapeutic and stray photon exposures throughout the entire body in individual patients.

PURPOSE: Photon radiotherapy techniques typically devote considerable attention to limiting the exposure of healthy tissues outside of the target volume. Numerous studies have shown, however, that commercial treatment planning systems (TPSs) significantly underestimate the absorbed dose outside of the treatment field. The purpose of this study was to test the feasibility of quickly and accurately calculating the total absorbed dose to the whole body from photon radiotherapy in individual patients. METHODS: We created an extended TPS by implementing a physics-based analytical model for the absorbed dose from stray photons during photon therapy into a research TPS. We configured and validated the extended TPS using measurements of 6- and 15-MV photon beams in water-box and anthropomorphic phantoms. We characterized the additional computation time required for therapeutic and stray dose calculations in a 44 × 30 × 180 cm3 water-box phantom. RESULTS: The extended TPS achieved superior dosimetric accuracy compared to the research TPS in both water and anthropomorphic phantoms, especially outside of the primary treatment field. In the anthropomorphic phantom, the extended TPS increased the generalized gamma index passing rate by a factor of 10 and decreased the median dosimetric discrepancy in the out-of-field region by a factor of 26. The extended TPS achieved an average discrepancy <1% in and near the treatment field and <1 mGy/Gy far from the treatment field in the anthropomorphic phantom. Characterization of computation time revealed that on average, the extended TPS only required 7% longer than the research TPS to calculate the total absorbed dose. CONCLUSIONS: The results of this work suggest that it is feasible to quickly and accurately calculate whole-body doses inside and outside of the therapeutic treatment field in individual patients on a routine basis using physics-based analytical dose models. This additional capability enables a more personalized approach to minimizing the risk of radiogenic late effects, such as second cancer and cardiac toxicity, as part of the treatment planning process.

Influence of beam quality on reference dosimetry correction factors in magnetic resonance guided radiation therapy.

Correction factors for reference dosimetry in magnetic resonance (MR) imaging-guided radiation therapy ( k B → , M , Q ) are often determined in setups that combine a conventional 6 MV linac with an electromagnet. This study investigated whether results based on these measurements were applicable for a 7 MV MR-linac using Monte Carlo simulations. For a Farmer-type ionization chamber, k B → , M , Q was assessed for different tissue-phantom ratios ( TPR 20 , 10 ). k B → , M , Q differed by 0.0029 ( 43 ) between TPR 20 , 10 = 0.6790 ( 23 ) (6 MV linac) and TPR 20 , 10 = 0.7028 ( 14 ) (7 MV MR-linac) at 1.5 T . The agreement was best in an orientation in which the secondary electrons were deflected to the stem of the ionization chamber.

[Detector Based Determination of Water Absorbed Dose According to DIN 6800 Teil 1: Suggestion for an Extension of the Fundamental Formalism]. / Ermittlung der Wasser-Energiedosis nach der Sondenmethode gemäß DIN 6800 Teil 1: Vorschlag für eine Erweiterung der Grundgleichung.

For any detector to be used for the determination of absorbed dose at the point of measurement in water a basic equation is required to convert the reading of the detector into absorbed dose in water. The German DIN 6800 part 1 provides a general formalism for that. A further differentiated formalism applicable to photon dosimetry is suggested in this work. This modified formalism presents the two following still general and at the same time fundamental properties of any dosimetry detector: a) a clear distinction of correction factors with respect to the physical processes involved during the measurement, and b) the fact that the process of energy absorption in the detector is determined by the spectral distribution of the fluence of the secondary charged particles. It is concluded that based on the modified formalism and knowing this spectral distribution within the detector a general method is available with benefits for ionization chambers as well as for any other dosimetry detector and which is applicable at reference as well as non-reference conditions without any preconditions.

A physics-based analytical model of absorbed dose from primary, leakage, and scattered photons from megavoltage radiotherapy with MLCs.

A burgeoning population of cancer survivors is at risk of late health effects following radiation therapy including second cancers, with the majority of these cancers occurring outside of the treatment volume of the primary cancer. Commercial radiotherapy treatment planning systems underestimate the out-of-field dose. Previous analytical models of out-of-field dose have assumed radial symmetry and/or approximated the dimensions of collimators as semi-infinite planes. The purpose of this work was to develop a physics-based analytical model of total absorbed dose from primary, scattered, and leakage radiation for square fields from a 6 MV beam at any arbitrary point in a phantom, including in-plane, cross-plane, and out-of-plane locations. The model includes the essential physics of radiation transport through beam-limiting-devices including rounded edges of MLC leaves. The model agreed well with measurements and Monte Carlo simulations of absorbed dose in a water-box phantom and was validated for field-sizes ranging from 2 [Formula: see text] 2 to 20 [Formula: see text] 20 cm2. The signed and unsigned average percent differences were [Formula: see text] and 15.9%, respectively, for all points and field-sizes considered. An extended gamma index analysis reveals a 92% pass rate with criteria of 3 mm distance-to-agreement, 3% relative dose difference in-field, and 3 mGy Gy-1 absolute dose difference out-of-field. The average wall-clock time to calculate dose to one million points was 3.3 min. These results suggest that it is feasible to calculate absorbed dose from both therapeutic and stray radiation using physics-based analytical models with good accuracy, thus overcoming a major obstacle to routinely assessing exposures. Additionally, this work demonstrates the importance of relaxing certain simplifications such as assuming a radially symmetric stray-dose distribution. Because the model is physics-based and may be reconfigured according to the dimensions of beam-limiting-devices, adapting it to other treatment units should be straight forward. Uses for such a model include clinical and research applications, such as clinical trials and epidemiological studies.

A finite element method for the determination of the relative response of ionization chambers in MR-linacs: simulation and experimental validation up to 1.5 T.

In magnetic resonance (MR) guided radiotherapy, the magnetic field-dependent change in the dose response of ionization chambers is typically included by means of a correction factor [Formula: see text]. This factor can be determined experimentally or calculated by means of Monte Carlo (MC) simulations. To date, a small number of experimental values for [Formula: see text] at magnetic flux densities above 1.2 T have been available to benchmark these simulations. Furthermore, MC simulations of the dose response of ionization chambers in magnetic fields (where such simulations are based on manufacturer blueprints) have been shown to converge with results that deviate considerably from experimental values for orientations where the magnetic field is perpendicular to the axis of the ionization chamber and the influence of the magnetic field is largest. In this work, [Formula: see text] was simulated for a PTW 30013 Farmer ionization chamber using an approach based on finite element simulations. First, the electrical field inside the ionization chamber was simulated using finite element methods. The collecting volume of the ionization was not defined in terms of the physical dimensions of the detector but in terms of the simulated electrical field lines inside the chamber. Then, an MC simulation of the dose response of a Farmer type chamber (PTW 30013) was performed using EGSnrc with a dedicated package to consider the effect of the magnetic field. In the second part, [Formula: see text] was determined experimentally for two different PTW 30013 ionization chambers for a range of magnetic flux densities between B = 0 and 1.5 T, covering the range of commercially available MR-linacs. In the perpendicular orientation, the maximum difference between the simulated values for [Formula: see text] and the experimental values for [Formula: see text] was 0.31(30)% and the minimum difference was 0.02(24)%. For the PTW 30013 ionization chambers, the experimental values for [Formula: see text] were 0.9679(1) and 0.9681(1) for a magnetic flux density of 1.5 T. The value resulting from the simulation was 0.967(3). The comparison of the correction factors simulated using this new approach with the experimental values determined in this study shows excellent agreement for all magnetic flux densities up to 1.5 T. Integrating the explicit simulation of the collection volume inside the ionization chambers into the MC simulation model significantly improves simulations of the chamber response in magnetic fields. The results presented suggest that intra-type variations for [Formula: see text] may be neglectable for ionization chambers of the PTW 30013 type.