ACPSEM position paper: dosimetry for magnetic resonance imaging linear accelerators.

Consistency and clear guidelines on dosimetry are essential for accurate and precise dosimetry, to ensure the best patient outcomes and to allow direct dose comparison across different centres. Magnetic Resonance Imaging Linac (MRI-linac) systems have recently been introduced to Australasian clinics. This report provides recommendations on reference dosimetry measurements for MRI-linacs on behalf of the Australiasian College of Physical Scientists and Engineers in Medicine (ACPSEM) MRI-linac working group. There are two configurations considered for MRI-linacs, perpendicular and parallel, referring to the relative direction of the magnetic field and radiation beam, with different impacts on dose deposition in a medium. These recommendations focus on ion chambers which are most commonly used in the clinic for reference dosimetry. Water phantoms must be MR safe or conditional and practical limitations on phantom set-up must be considered. Solid phantoms are not advised for reference dosimetry. For reference dosimetry, IAEA TRS-398 recommendations cannot be followed completely due to physical differences between conventional linac and MRI-linac systems. Manufacturers' advice on reference conditions should be followed. Beam quality specification of TPR20,10 is recommended. The configuration of the central axis of the ion chamber relative to the magnetic field and radiation beam impacts the chamber response and must be considered carefully. Recommended corrections to delivered dose are [Formula: see text], a correction for beam quality and [Formula: see text], for the impact of the magnetic field on dosimeter response in the magnetic field. Literature based values for [Formula: see text] are given. It is important to note that this is a developing field and these recommendations should be used together with a review of current literature.

Experimental characterisation of the magnetic field correction factor,kB⃗,for Roos chambers in a parallel MRI-linac.

Objective.Reference dosimetry on an MRI-linac requires a chamber specific magnetic field correction factor,kB⃗.This work aims to measure the correction factor for a parallel plate chamber on a parallel MRI-linac.Approach.kB⃗is defined as the ratio of the absorbed dose to water calibration coefficient in the presence of the magnetic field,ND,wB⃗relative to that under 0 T conditions,ND,w0T.kB⃗was measured via aND,wtransfer to a field chamber at each magnetic field strength from a chamber with knownND,wandkB⃗.This was achieved on the parallel MRI-linac by moving the measurement set-up between a high magnetic field strength region at the MRI-isocentre and a low magnetic field strength region at the end of the bore whilst maintaining consistent set-up and scatter conditions. Three PTW 34001 Roos chambers were investigated as well as a PTW 30013 Farmer used to validate methodology.Main Results.The beam quality used for the measurements ofkB⃗wasTPR20/10 = 0.632. ThekB⃗for the PTW Farmer chamber at 1 T on a parallel MRI-linac was 0.993 ± 0.013 (k = 1). The averagekB⃗factor measured for the three Roos chambers on a 1 T parallel MRI-linac was 0.999 ± 0.014 (k = 1).Significance.The results presented are the first measurements ofkB⃗for a Roos chamber on a parallel MRI-linac. The Roos chamber results demonstrate the potential for the chamber as a reference dosimeter in parallel MRI-linacs.

Dosimetric Optimization and Commissioning of a High Field Inline MRI-Linac.

Purpose: Unique characteristics of MRI-linac systems and mutual interactions between their components pose specific challenges for their commissioning and quality assurance. The Australian MRI-linac is a prototype system which explores the inline orientation, with radiation beam parallel to the main magnetic field. The aim of this work was to commission the radiation-related aspects of this system for its application in clinical treatments. Methods: Physical alignment of the radiation beam to the magnetic field was fine-tuned and magnetic shielding of the radiation head was designed to achieve optimal beam characteristics. These steps were guided by investigative measurements of the beam properties. Subsequently, machine performance was benchmarked against the requirements of the IEC60976/77 standards. Finally, the geometric and dosimetric data was acquired, following the AAPM Task Group 106 recommendations, to characterize the beam for modeling in the treatment planning system and with Monte Carlo simulations. The magnetic field effects on the dose deposition and on the detector response have been taken into account and issues specific to the inline design have been highlighted. Results: Alignment of the radiation beam axis and the imaging isocentre within 2 mm tolerance was obtained. The system was commissioned at two source-to-isocentre distances (SIDs): 2.4 and 1.8 m. Reproducibility and proportionality of the dose monitoring system met IEC criteria at the larger SID but slightly exceeded it at the shorter SID. Profile symmetry remained under 103% for the fields up to ~34 × 34 and 21 × 21 cm2 at the larger and shorter SID, respectively. No penumbra asymmetry, characteristic for transverse systems, was observed. The electron focusing effect, which results in high entrance doses on central axis, was quantified and methods to minimize it have been investigated. Conclusion: Methods were developed and employed to investigate and quantify the dosimetric properties of an inline MRI-Linac system. The Australian MRI-linac system has been fine-tuned in terms of beam properties and commissioned, constituting a key step toward the application of inline MRI-linacs for patient treatments.

High resolution silicon array detector implementation in an inline MRI-linac.

PURPOSE: Dynamic dosimaging is a concept whereby a detector in motion is tracked with magnetic resonance imaging (MRI) to validate the amount and position of dose in a radiation therapy treatment on an MRI-linac. This work takes steps toward the realization of dynamic dosimaging with the novel high resolution silicon array detector: MagicPlate-512 (M512). The performance of the M512 was assessed in a 1.0 T inline MRI-linac, without simultaneous imaging and then during an imaging sequence, both during dosimetry. MR images were acquired to determine the effect of the detector and its components on image quality. METHODS: Beam profiles were measured using the M512 on the Australian MRI-Linac and a comparison made with Gafchromic EBT3 film to investigate any intrinsic magnetic field effects in the silicon. The M512 has 512 sensitive volumes, each 0.5 × 0.5 × 0.037 mm3 in dimension, organized in a two-dimensional array. Small field sizes up to 4.2 × 3.8 cm2 were investigated in both solid water and then solid lung phantoms. Beam profiles taken at 1.0 T were compared to 0 T conditions, and also to profiles taken during a gradient echo (GRE) imaging sequence. Differences in 80%-20% penumbral width and full width at half maximum (FWHM) were investigated. Localizer MR images were acquired of the detector adjacent to a water phantom. RESULTS: Good agreement was observed between the M512 and film, with average differences in penumbral width and FWHM of <1 mm in the absence of the imaging sequence. Concurrent imaging widened the penumbra by up to 1.2 mm due to RF noise affecting the detector; film profiles were unchanged. Magnetic resonance images were affected by noise, in particular, due to the large amount of aluminum present, as well as from the USB cable, which acted as an antenna. Unfortunately, due to these issues, suitable dynamic dose imaging was not achieved with the current M512/phantom configuration and the MRI-linac. However, progress was made toward achieving this goal for future work. CONCLUSIONS: The M512 silicon array detector successfully measured high-resolution beam profiles in agreement with Gafchromic film to within an average of <1 mm on the first MRI-linac in Australia. More effective noise reduction will be required for the achievement of dynamic dosimaging in the future.

Technical Note: Experimental characterization of the dose deposition in parallel MRI-linacs at various magnetic field strengths.

PURPOSE: Dose deposition measurements for parallel MRI-linacs have previously only shown comparisons between 0 T and a single available magnetic field. The Australian MRI-Linac consists of a magnet coupled with a dual energy linear accelerator and a 120 leaf Multi-Leaf Collimator with the radiation beam parallel to the magnetic field. Two different magnets, with field strengths of 1 and 1.5 T, were used during prototyping. This work aims to characterize the impact of the magnetic field at 1 and 1.5 T on dose deposition, possible by comparing dosimetry measured at both magnetic field strengths to measurements without the magnetic field. METHODS: Dose deposition measurements focused on a comparison of beam quality (TPR20/10 ), PDD, profiles at various depths, surface doses, and field size output factors. Measurements were acquired at 0, 1, and 1.5 T. Beam quality was measured using an ion chamber in solid water at isocenter with appropriate TPR20/10 buildup. PDDs and profiles were acquired via EBT3 film placed in solid water either parallel or perpendicular to the radiation beam. Films at surface were used to determine surface dose. Output factors were measured in solid water using an ion chamber at isocenter with 10 cm solid water buildup. RESULTS: Beam quality was within ±0.5% of the 0 T value for the 1 and 1.5 T magnetic field strengths. PDDs and profiles showed agreement for the three magnetic field strengths at depths beyond 20 mm. Deposited dose increased at shallower depths due to electron focusing. Output factors showed agreement within 1%. CONCLUSION: Dose deposition at depth for a parallel MRI-linac was not significantly impacted by either a 1 or 1.5 T magnetic field. PDDs and profiles at shallow depths and surface dose measurements showed significant differences between 0, 1, and 1.5 T due to electron focusing.

Technical Note: Penumbral width trimming in solid lung dose profiles for 0.9 and 1.5 T MRI-Linac prototypes.

PURPOSE: Longitudinal magnetic fields narrow beam penumbra and tighten lateral spread of secondary electrons in air cavities, including lung tissue. Gafchromic® EBT3 film was used to investigate differences between penumbra in solid water and solid lung, without a magnetic field (0 T) and with two field strengths (0.9 and 1.5 T). METHODS: The first prototype of the Australian MRI-linac consisted of a 1.5 T Siemens Sonata MRI and Varian industrial linatron (nominal 4 MV). The second prototype replaced the Sonata with a 1.0 T Agilent split-bore magnet. Measurements were completed at 0.9 T to maintain the same source-to-surface distance between set-ups. Gammex-rmi® solid water with 50 mm of CIRS solid lung inserted as a lung cavity was positioned inside each magnet. This was compared to the same set-up with solid water only, where film measurements were completed at solid water equivalent depths corresponding to entrance interface/mid/exit interface positions of solid lung from the first set-up. Multileaf collimator (MLC)-defined field sizes were set to 3 × 3 cm2 and 10 × 10 cm2 . The 80%-20% penumbral width was determined. RESULTS: Under 1.5 T conditions, penumbra narrowing occurred up to 4.4 ± 0.1 mm compared to 0 T. As expected, the effect was less for 0.9 T, which resulted in a maximum narrowing of 2.5 ± 0.1 mm. Exit profile penumbra were more affected than entrance penumbra by up to 2.6 ± 0.2 mm. The 1.5 T field brought the solid water and lung penumbral widths more into alignment by a maximum difference of 0.4 ± 0.1 mm. CONCLUSIONS: The trimming of penumbral widths due to magnetic fields in solid water and lung was demonstrated and compared to 0 T. The 0.9 and 1.5 T field trimmed the penumbra by up to 2.5 ± 0.1 mm and 4.4 ± 0.1 mm respectively.

Initial experiments with gel-water: towards MRI-linac dosimetry and imaging.

Tracking the position of a moving radiation detector in time and space during data acquisition can replicate 4D image-guided radiotherapy (4DIGRT). Magnetic resonance imaging (MRI)-linacs need MRI-visible detectors to achieve this, however, imaging solid phantoms is an issue. Hence, gel-water, a material that provides signal for MRI-visibility, and which will in future work, replace solid water for an MRI-linac 4DIGRT quality assurance tool, is discussed. MR and CT images of gel-water were acquired for visualisation and electron density verification. Characterisation of gel-water at 0 T was compared to Gammex-RMI solid water, using MagicPlate-512 (M512) and RMI Attix chamber; this included percentage depth dose, tissue-phantom ratio (TPR20/10), tissue-maximum ratio (TMR), profiles, output factors, and a gamma analysis to investigate field penumbral differences. MR images of a non-powered detector in gel-water demonstrated detector visualisation. The CT-determined gel-water electron density agreed with the calculated value of 1.01. Gel-water depth dose data demonstrated a maximum deviation of 0.7% from solid water for M512 and 2.4% for the Attix chamber, and by 2.1% for TPR20/10 and 1.0% for TMR. FWHM and output factor differences between materials were ≤0.3 and ≤1.4%. M512 data passed gamma analysis with 100% within 2%, 2 mm tolerance for multileaf collimator defined fields. Gel-water was shown to be tissue-equivalent for dosimetry and a feasible option to replace solid water.

A phantom assessment of achievable contouring concordance across multiple treatment planning systems.

In this paper, the highest level of inter- and intra-observer conformity achievable with different treatment planning systems (TPSs), contouring tools, shapes, and sites have been established for metrics including the Dice similarity coefficient (DICE) and Hausdorff Distance. High conformity values, e.g. DICE(Breast_Shape)=0.99±0.01, were achieved. Decreasing image resolution decreased contouring conformity.

Utilising pseudo-CT data for dose calculation and plan optimization in adaptive radiotherapy.

To quantify the dose calculation error and resulting optimization uncertainty caused by performing inverse treatment planning on inaccurate electron density data (pseudo-CT) as needed for adaptive radiotherapy and Magnetic Resonance Imaging (MRI) based treatment planning. Planning Computer Tomography (CT) data from 10 cervix cancer patients was used to generate 4 pseudo-CT data sets. Each pseudo-CT was created based on an available method of assigning electron density to an anatomic image. An inversely modulated radiotherapy (IMRT) plan was developed on each planning CT. The dose calculation error caused by each pseudo-CT data set was quantified by comparing the dose calculated each pseudo-CT data set with that calculated on the original planning CT for the same IMRT plan. The optimization uncertainty introduced by the dose calculation error was quantified by re-optimizing the same optimization parameters on each pseudo-CT data set and comparing against the original planning CT. Dose differences were quantified by assessing the Equivalent Uniform Dose (EUD) for targets and relevant organs at risk. Across all pseudo-CT data sets and all organs, the absolute mean dose calculation error was 0.2 Gy, and was within 2 % of the prescription dose in 98.5 % of cases. Then absolute mean optimisation error was 0.3 Gy EUD, indicating that that inverse optimisation is impacted by the dose calculation error. However, the additional uncertainty introduced to plan optimisation is small compared the sources of variation which already exist. Use of inaccurate electron density data for inverse treatment planning results in a dose calculation error, which in turn introduces additional uncertainty into the plan optimization process. In this study, we showed that both of these effects are clinically acceptable for cervix cancer patients using four different pseudo-CT data sets. Dose calculation and inverse optimization on pseudo-CT is feasible for this patient cohort.

Kilovoltage cone-beam CT imaging dose during breast radiotherapy: a dose comparison between a left and right breast setup.

The purpose of this study was to investigate the delivered dose from a kilovoltage cone-beam computed tomography (kV-CBCT) acquired in breast treatment position for a left and right breast setup. The dose was measured with thermoluminescent dosimeters positioned within a female anthropomorphic phantom at organ locations. Imaging was performed on an Elekta Synergy XVI system with the phantom setup on a breast board. The image protocol involved 120kVp, 140mAs, and a 270° arc rotation clockwise 0° to 270° for the left breast setup and 270° to 180° for the right breast setup (maximum arc rotations possible). The dose delivered to the left breast, right breast, and heart was 5.1mGy, 3.9mGy, and 4.0mGy for the left breast setup kV-CBCT, and 6.4mGy, 6.0mGy, and 4.8mGy for the right breast setup kV-CBCT, respectively. The rotation arc of the kV-CBCT influenced the dose delivered, with the right breast setup kV-CBCT found to deliver a dose of up to 4mGy or 105% higher to the treated breast's surface in comparison with the left breast setup. This is attributed to the kV-CBCT source being more proximal to the anterior of the phantom for a right breast setup, whereas the source is more proximal to the posterior of the patient for a left-side scan.