Magnetic Resonance Imaging linear-accelerator (MRI-linac) equipment has recently been introduced to multiple centres in Australia and New Zealand. MRI equipment creates hazards for staff, patients and others in the MR environment; these hazards must be well understood, and risks managed by a system of environmental controls, written procedures and a trained workforce. While MRI-linac hazards are similar to the diagnostic paradigm, the equipment, workforce and environment are sufficiently different that additional safety guidance is warranted. In 2019 the Australasian College of Physical Scientists and Engineers in Medicine (ACPSEM) formed the Magnetic Resonance Imaging Linear-Accelerator Working Group (MRILWG) to support the safe clinical introduction and optimal use of MR-guided radiation therapy treatment units. This Position Paper is intended to provide safety guidance and education for Medical Physicists and others planning for and working with MRI-linac technology. This document summarises MRI-linac hazards and describes particular effects which arise from the combination of strong magnetic fields with an external radiation treatment beam. This document also provides guidance on safety governance and training, and recommends a system of hazard management tailored to the MRI-linac environment, ancillary equipment, and workforce.

Magnetic resonance-guided radiotherapy technology is relatively new and commissioning publications, quality assurance (QA) protocols and commercial products are limited. This work provides guidance for implementation measurements that may be performed on the Elekta Unity MR-Linac (Elekta, Stockholm, Sweden). Adaptations of vendor supplied phantoms facilitated determination of gantry angle accuracy and linac isocentre, whereas in-house developed phantoms were used for end-to-end testing and anterior coil attenuation measurements. Third-party devices were used for measuring beam quality, reference dosimetry and during treatment plan commissioning; however, due to several challenges, variations on standard techniques were required. Gantry angle accuracy was within 0.1°, confirmed with pixel intensity profiles, and MV isocentre diameter was < 0.5 mm. Anterior coil attenuation was approximately 0.6%. Beam quality as determined by TPR20,10 was 0.705 ± 0.001, in agreement with treatment planning system (TPS) calculations, and gamma comparison against the TPS for a 22.0 × 22.0 cm2 field was above 95.0% (2.0%, 2.0 mm). Machine output was 1.000 ± 0.002 Gy per 100 MU, depth 5.0 cm. During treatment plan commissioning, sub-standard results indicated issues with machine behaviour. Once rectified, gamma comparisons were above 95.0% (2.0%, 2.0 mm). Centres which may not have access to specialized equipment can use in-house developed phantoms, or adapt those supplied by the vendor, to perform commissioning work and confirm operation of the MRL within published tolerances. The plan QA techniques used in this work can highlight issues with machine behaviour when appropriate gamma criteria are set.

MR-guided radiotherapy on hybrid MR-Linacs exploits the excellent soft-tissue contrast of MRI to deliver daily adaptive precision radiotherapy. Geometric fidelity and long-term stability of MRI components are essential, but their longitudinal performance under daily exposure to scattered ionizing radiation is unknown. We report on longitudinal stability of periodic MRI QA on eight clinical 1.5T MR-Linac systems. We provided measurement instructions for periodic MRI QA and received data from seven different centers within the Elekta MR-Linac consortium, which contributed data over acquisition periods ranging from 3–24 months. We tested B0 and B1 homogeneity using a 37 cm diameter cylindrical phantom, which was measured monthly in axial orientation, supplemented by quarterly sagittal and coronal acquisitions. We report average, standard deviation and peak-to-peak variation (99th-1st percentile) within a region of interest (ROI) of 35 cm diameter. Dependence of B0 on the gantry angle and gradient non-linearity were tested quarterly. We analyzed the longitudinal stability of selected metrics of the vendor-provided periodic image quality tests. We found high temporal stability of B0 and B1 measurements and good agreement between different MR-Linac systems. For all measurements, the standard deviation of B0 within the analyzed ROI was below 0.66/0.33/0.33 ppm for axial/sagittal/coronal orientation. The average standard deviation of the ratio between actual and nominal flip angle was 0.022/0.100/0.088 for axial/sagittal/coronal orientation. Systems exhibited distinctively different gantry angle dependencies of B0, with sensitivities of B0 to the gantry angle differing by factors of up to two between systems. Gradient non-linearity analysis yielded average radii of 172 and 242 mm for which 98% of the phantom markers had deviations below 1 and 2 mm, respectively. All analyzed periodic image quality tests were passed, but major events including a body coil replacement and ramp down were apparent in the time series. Overall we found very similar performance of the tested systems and our results could inform the implementation of MR imaging QA for MR-Linacs. While we found differences of the gantry angle dependence of B0 between systems, the high temporal stability found for all tests is a foundation for stereotactic radiotherapy and multi-center clinical trials involving quantitative MRI.

We investigate the properties of a light emitting diode (LED) flatbed scanner for use with EBT3 and EBT-XD film types in a clinical radiochromic film (RCF) dosimetry program with modern treatment techniques. The flatbed scanner was characterised in terms of lateral and longitudinal response, X–Y scaling integrity, scanning reproducibility, scanner warm up dependence and film orientation dependence. The preferred lateral response artefact (LRA) corrections are investigated for the LED light source. Supporting evidence is provided regarding the dose independent nature of the corrections while also providing results suggesting a potential film type independence. Results from 2D gamma analysis of four patient treatments were compared between the new 12000XL and existing 10000XL model. Lastly, a dose uncertainty analysis was performed for the film–scanner system combination. It may be concluded that the lateral response variation requires correction while the longitudinal response variation is insignificant. The linear scaling in the lateral and longitudinal directions are within 0.5% and the scanner reproducibility is stable. Scanner warm up dependence no longer exists, and effort should be made to maintain all film orientation in a study set within 15°. The LRA corrections are as reported substantially dose independent and there is evidence to support film type independence. Comparative gamma analysis of patient specific dose maps between the EPSON 10000XL (xenon fluorescent lamp) and 12000XL (LED) scanners showed that results are indistinguishable for both film types across the two scanner models when the necessary corrections are applied. Dose uncertainty is in agreement with the literature and can be kept below 3% with necessary corrections applied.

Accurate profiles of alkali spectral lines pressure broadened by collisions with molecular hydrogen and atomic helium perturbers are of crucial importance for the modeling of atmospheres of brown dwarfs. Calculations of these Lorentzian profiles are computationally demanding as they involve the integration of coupled second-order differential equations describing the colliding atoms out to large radial separations, where the solutions can be matched to a combination of the solutions for non-interacting atoms. This process must be repeated for a wide range of energies and relative angular momenta of the perturbers. We report on the use of two R-matrix packages, RPROP2 and FARM to solve these equations for collisions of atomic helium with Na and K, which are the dominant alkalies in brown dwarf spectra. The RPROP2 routine is extremely slow in todays terms but is simple to use and to modify where necessary. Conversely the FARM routine is extremely fast but considerably less transparent, incorporating a less computationally intensive potential--following propagator at intermediate radial separations where the potential is more slowly varying and an analytical, asymptotic solution fitting routine for large radial separations.

We report results for the Lorentzian profiles of the Li I, Na I and K I doublets and the Na I subordinate doublet broadened by helium perturbers for temperatures up to 3000 K. They have been obtained from a fully quantum-mechanical close-coupling description of the colliding atoms, the Baranger theory of line shapes and new ab initio potentials for the alkali–helium interaction. For all lines except the 769.9 nm K I line, the temperature dependence of the widths over the range 70 ⩽ T ⩽ 3000 K is accurately represented by the power law form w = aTbb with 0.38 < b < 0.43. The 769.9 nm K I line has this form for 500 ⩽ T ⩽ 3000 K with b having the higher value of 0.49. Although the shifts have a more complex temperature dependence, they all have the general feature of increasing with temperature above T ∼ 500 K apart from the 769.9 K I line whose shift decreases with temperature.