Monte Carlo calculation of detector perturbation and quality correction factors in a 1.5 T magnetic resonance guided radiation therapy small photon beams.

Objective.With future advances in magnetic resonance imaging-guided radiation therapy, small photon beams are expected to be included regularly in clinical treatments. This study provides physical insights on detector dose-response to multiple megavoltage photon beam sizes coupled to magnetic fields and determines optimal orientations for measurements.Approach.Monte Carlo simulations determine small-cavity detector (solid-state: PTW60012 and PTW60019, ionization chambers: PTW31010, PTW31021, and PTW31022) dose-responses in water to an Elekta Unity 7 MV FFF photon beam. Investigations are performed for field widths between 0.25 and 10 cm in four detector axis orientations with respect to the 1.5 T magnetic field and the photon beam. The magnetic field effect on the overall perturbation factor (PMC) accounting for the extracameral components, atomic composition, and density is quantified in each orientation. The density (Pρ) and volume averaging (Pvol) perturbation factors and quality correction factors (kQB,QfB,f) accounting for the magnetic field are also calculated in each orientation.Main results.Results show thatPvolremains the most significant perturbation both with and without magnetic fields. In most cases, the magnetic field effect onPvolis 1% or less. The magnetic field effect onPρis more significant on ionization chambers than on solid-state detectors. This effect increases up to 1.564 ± 0.001 with decreasing field size for chambers. On the contrary, the magnetic field effect on the extracameral perturbation factor is higher on solid-state detectors than on ionization chambers. For chambers, the magnetic field effect onPMCis only significant for field widths <1 cm, while, for solid-state detectors, this effect exhibits different trends with orientation, indicating that the beam incident angle and geometry play a crucial role.Significance.Solid-state detectors' dose-response is strongly affected by the magnetic field in all orientations. The magnetic field impact on ionization chamber response increases with decreasing field size. In general, ionization chambers yieldkQB,QfB,fcloser to unity, especially in orientations where the chamber axis is parallel to the magnetic field.

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 Billaset al(2020Phys. Med. Biol.65115001), 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 Pooteret al(2021Phys. Med. Biol.6605TR02), 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.

Eigencolor radiochromic film dosimetry.

PURPOSE: The goal of this work is to propose a new multichannel method correcting for systematic thickness disturbances and to evaluate its precision in relevant radiation dosimetry applications. METHODS: The eigencolor ratio technique is introduced and theoretically developed to provide a method correcting for thickness disturbances. The method is applied to EBT3 GafchromicTM film irradiated with cobalt-60 and 6 MV photon beams and digitized with an Epson 10000XL photo scanner. Dose profiles and output factors of different field sizes are measured and analyzed. Variance analysis of the previous method of Bouchard et al. ["On the characterization and uncertainty analysis of radiochromic film dosimetry" Med Phys. 2009;36:1931-1946] is adapted to the new approach. Uncertainties are predicted for relevant applications. RESULTS: Results show that systematic disturbances attributed to thickness variations are efficiently corrected. The method is shown efficient to identify and correct for dark spots which cause systematic errors in single-channel distributions. Applications of the method in the context of relative dosimetry yields standard uncertainties ranging between 0.8% and 1.9%, depending on the region of interest (ROI) size and the film irradiation. Variance analysis predicts that uncertainty levels between 0.3% and 0.6% are achievable with repeated measurements. Uncertainties are found to vary with absorbed dose and ROI size. CONCLUSIONS: The proposed multichannel method is efficient for accurate dosimetry, reaching uncertainty levels comparable to previous publications with EBT film. The method is also promising for applications beyond clinical QA, such as machine characterization and other advanced dosimetry applications.

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.

Small-cavity chamber dose response in megavoltage photon beams coupled to magnetic fields.

In MRgRT, dosimetry measurements are performed in the presence of magnetic fields. For high-resolution measurements, small-cavity ionization chambers are required. While Monte Carlo simulations are essential to determine dosimetry correction factors, models of small-chambers require careful validation with experimental measurements. The aim of this study is to characterize small-cavity chamber response coupled to magnetic fields. Small-cavity chambers (PTW31010, PTW31016, PTW31021 and PTW3022) are irradiated by a 6 MV photon beam for 9 magnetic field strengths between -1.5 T and +1.5 T. The chamber axis is orientated either parallel or perpendicular to the irradiation beam, with the magnetic field always perpendicular to the beam. MC simulations are performed in EGSnrc. The sensitive volume of the chambers is reduced to account for the inefficiency adjacent to the guard electrode (dead volume) based on COMSOL calculations of electric potentials. The magnetic field affects the chamber response by up to 4.1% and 4.5% in the parallel and perpendicular orientations, respectively, compared to no magnetic field. The maximal difference in dose response between experiments and simulations is up to 6.1% and 4.5% for parallel and perpendicular orientation, respectively. When the dead volume is removed, which accounts for the 15%-23% of the nominal volume, the difference, in most cases, is within the stated uncertainties. Nevertheless, for a particular chamber, the reduced nominal volume barely improved the agreement between the experimental and calculated relative response (4.53% to 4.13%). This disagreement may be due to the imperfect chamber geometry model, as was found from microCT images. A detailed uncertainty analysis is presented. The characterization of small-cavity ion chamber response coupled to magnetic fields is complex. Small differences between real and model chamber geometry that normally would be insignificant become an issue in the presence of magnetic fields. Accurate characterization of the nominal volume is essential for small-cavity ion chamber modelling.

Alanine dosimetry in strong magnetic fields: use as a transfer standard in MRI-guided radiotherapy.

Reference dosimetry in the presence of a strong magnetic field is challenging. Ionisation chambers have shown to be strongly affected by magnetic fields. There is a need for robust and stable detectors in MRI-guided radiotherapy (MRIgRT). This study investigates the behaviour of the alanine dosimeter in magnetic fields and assesses its suitability to act as a reference detector in MRIgRT. Alanine pellets were loaded in a waterproof holder, placed in an electromagnet and irradiated by 60Co and 6 MV and 8 MV linac beams over a range of magnetic flux densities. Monte Carlo simulations were performed to calculate the absorbed dose, to water and to alanine, with and without magnetic fields. Combining measurements with simulations, the effect of magnetic fields on alanine response was quantified and a correction factor for the presence of magnetic fields on alanine was determined. This study finds that the response of alanine to ionising radiation is modified when the irradiation is in the presence of a magnetic field. The effect is energy independent and may increase the alanine/electron paramagnetic resonance (EPR) signal by 0.2% at 0.35 T and 0.7% at 1.5 T. In alanine dosimetry for MRIgRT, this effect, if left uncorrected, would lead to an overestimate of dose. Accordingly, a correction factor, [Formula: see text], is defined. Values are obtained for this correction as a function of magnetic flux density, with a standard uncertainty which depends on the magnetic field and is 0.6% or less. The strong magnetic field has a measurable effect on alanine dosimetry. For alanine which is used to measure absorbed dose to water in a strong magnetic field, but which has been calibrated in the absence of a magnetic field, a small correction to the reported dose is required. With the inclusion of this correction, alanine/EPR is a suitable reference dosimeter for measurements in MRIgRT.

The effect of magnetic field strength on the response of Gafchromic EBT-3 film.

With the advent of MRI-guided radiotherapy, the suitability of commercially available radiation dose detectors needs to be assessed. The aim of this study was to investigate the effect of the magnetic field (B-field) on the response of the Gafchromic EBT-3 films. Moreover, as an independent study, we contribute to clarifying the inconsistency of the results of recent published studies, on the effect of B-field on the sensitivity of Gafchromic films. A 60Co beam was used to irradiate film samples in an electromagnet. An in-house PMMA phantom was designed to fit in the 5 cm gap between the two poles of the magnet. The phantom consists of two symmetrical plates where a film can be inserted. The absorbed dose rate of the 60Co beam for zero B-field was measured using alanine pellets in a Farmer-type holder. A 12-point response curve was created, representing [Formula: see text] as a function of dose, for each of five different B-field strengths (0 T to 2 T). This study finds that there is at most a small effect of the magnetic field on the response of EBT-3 film. In terms of netOD (red channel) the change in response varied from ‒0.0011 at 0.5 T to 0.0045 at 2.0 T, with a standard uncertainty of 0.0030. If uncorrected, this would lead to an error in film-measured dose, for the red channel, of 2.4% at 2 T, with a standard uncertainty on dose of 1.4%. Results are also presented for B-field strengths of 0.5 T, 1 T and 1.5 T, which are all zero within the measurement uncertainty. Comparison between other studies is also presented. Considering the small change on dose determined with EBT-3 when irradiated under the presence of B-field and taking into account the overall uncertainty in dosimetry using EBT-3 film achieved in this work, EBT-3 is assessed to be a suitable detector for relative and absolute dosimetry, with appropriate corrections, in MRI-guided radiotherapy. The results of the current work also elucidate the inconsistency on the reports from previous studies and demonstrate the necessity of similar investigations by independent teams, especially if the existing results may be in conflict.

Investigating the effect of a magnetic field on dose distributions at phantom-air interfaces using PRESAGE® 3D dosimeter and Monte Carlo simulations.

Dosimetric quality assurance (QA) of the new Elekta Unity (MR-linac) will differ from the QA performed of a conventional linac due to the constant magnetic field, which creates an electron return effect (ERE). In this work we aim to validate PRESAGE® dosimetry in a transverse magnetic field, and assess its use to validate the research version of the Monaco TPS of the MR-linac. Cylindrical samples of PRESAGE® 3D dosimeter separated by an air gap were irradiated with a cobalt-60 unit, while placed between the poles of an electromagnet at 0.5 T and 1.5 T. This set-up was simulated in EGSnrc/Cavity Monte Carlo (MC) code and relative dose distributions were compared with measurements using 1D and 2D gamma criteria of 3% and 1.5 mm. The irradiation conditions were adapted for the MR-linac and compared with Monaco TPS simulations. Measured and EGSnrc/Cavity simulated profiles showed good agreement with a gamma passing rate of 99.9% for 0.5 T and 99.8% for 1.5 T. Measurements on the MR-linac also compared well with Monaco TPS simulations, with a gamma passing rate of 98.4% at 1.5 T. Results demonstrated that PRESAGE® can accurately measure dose and detect the ERE, encouraging its use as a QA tool to validate the Monaco TPS of the MR-linac for clinically relevant dose distributions at tissue-air boundaries.