Dosimetric properties of a prototype high field parallel MRI-Linac

Magnetic resonance imaging (MRI) linear accelerators (linacs) are a new treatment modality combining the imaging of MRI and the therapy of linacs. The Australian MRI-linac is a 1 T bespoke MRI coupled parallel with a linac. This body of work dosimetrically evaluates the properties of a prototype high field parallel MRI-linac as part of the goal of treating a patient. Accurate determination of dose is required to ensure good patient outcomes and minimise the potential of harmful side effects from radiotherapy treatments. For MRI-linacs, the magnetic field can impact both the dose deposition and the ion chamber response, necessitating a magnetic field correction factor to be applied during dose output measurements. Chapter 3 reviews the literature and provides recommendations to the Australasian College of Physical Scientists in Medicine for determination of dose on MRI-linacs. Recommendations include: specifying ion chambers and their associated magnetic field correction factors, a dose calculation system and link back to the primary standards laboratory, measurement set-up, beam quality, and configuration of the ion chamber, primary magnetic field and radiation beam. Dosimetric evaluation of a linac involves a determination of the dose distribution. Chapter 4 presents measured comparisons of the percentage depth dose, profiles, beam quality and output factors at 0, 1 and 1.5 T for a high field parallel MRI-linac. Dose deposition shows a significant increase in dose at the surface and no difference at depth in the magnetic field relative to a 0 T field. Beam quality and output factors were consistent across the magnetic field strengths. Accurate dose calibration is an essential part of the dosimetric evaluation. Chapter 5 presents a method for measuring the magnetic field correction factor by moving the linac and dosimeter set-up between the low and high magnetic field strength. Correction factors were 0.993 ± 0.013 and 0.999 ± 0.014 for a cylindrical ion chamber and Roos chamber respectively. The results were the first measurements of the correction factor for a Roos chamber and demonstrate the potential for use as a reference dosimeter. Measurements of magnetic field correction factors typically require specialised equipment and expertise. A method using a microDiamond detector to cross-calibrate an ion chamber between a conventional linac and MRI-linac was investigated in Chapter 6. Correction factors were measured on both a 1 T parallel MRI-linac and a 1.5 T perpendicular MRI-linac and showed good agreement with previously published results. Dosimetric evaluation showed potential safety issues for patient treatments. Chapter 7 investigates additional dose to the patient due to differences in dose distribution and additional imaging for a clinical trial. In vivo skin measurements on a phantom showed dose within clinical tolerances and the risk to patients from additional dose was low. This thesis has dosimetrically characterised a high field parallel MRI-linac and demonstrated the feasibility of safe treatments on a clinical trial

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. DICEBreast_Shape = 0.99 ± 0.01, were achieved. Decreasing image resolution decreased contouring conformity

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

© Copyright © 2020 Jelen, Dong, Begg, Roberts, Whelan, Keall and Liney. 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

Ion chamber magnetic field correction factors measured via microDiamond cross-calibration from a conventional linac to MRI-linac

Magnetic field correction factors are required for performing reference dosimetry on Magnetic Resonance Imaging Linear accelerators (MRI-linacs). Methods for measuring magnetic field correction factors usually require specialized equipment and expertise. Our work investigated the use of a microDiamond detector to cross-calibrate an ion chamber between a conventional linac and MRI-linac as a method to measure ion chamber magnetic field correction factors for the MRI-linac. Ratios of the microDiamond and ion chamber were measured on a conventional linac, parallel MRI-linac at 0 T, parallel MRI-linac at 1 T and perpendicular MRI-linac at 1.5 T. The beam quality dependence of the microDiamond was investigated by comparing the measurements on the conventional linac and parallel MRI-linac at 0 T. The magnetic field dependence of the microDiamond was investigated comparing the measurements on a parallel MRI-linac at 0 and 1 T. The ion chamber magnetic field correction factors were calculated by comparing the conventional linac and parallel MRI-linac at 1 T and the conventional linac and perpendicular MRI-linac at 1.5 T for the parallel and perpendicular factors respectively. The FC65-G and PTW30013 ion chambers were investigated. For a parallel MRI-linac, with a beam quality of (Formula presented.) = 0.632, we measured magnetic field correction factors of 0.988 ± 0.016 (k = 2) and 0.987 ± 0.016 (k = 2) for a FC65-G and PTW30013 respectively, where k is the coverage factor. For a perpendicular MRI-linac, with a beam quality of (Formula presented.) = 0.701, we measured magnetic field correction factors of 0.995 ± 0.020 (k = 2) and 0.983 ± 0.020 (k = 2) for a FC65-G and PTW30013 respectively. The results showed agreement with previously published work which used different approaches. Our work demonstrates the use of a microDiamond to calculate the ion chamber magnetic field correction factor using measurements on a conventional linac and MRI-linac. The high level of uncertainty in our results means the method at present can only be used for validation of magnetic field correction factors

Experimental characterisation of the magnetic field correction factor, k B →, for Roos chambers in a parallel MRI-linac

Objective. Reference dosimetry on an MRI-linac requires a chamber specific magnetic field correction factor, kB -. This work aims to measure the correction factor for a parallel plate chamber on a parallel MRI-linac. Approach. kB→is defined as the ratio of the absorbed dose to water calibration coefficient in the presence of the magnetic field, ND,wB→relative to that under 0 T conditions, ND,w0T. kB→was measured via a ND,w transfer to a field chamber at each magnetic field strength from a chamber with known ND,w and kB -. 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 of kB→was TPR 20/10 = 0.632. The kB→for the PTW Farmer chamber at 1 T on a parallel MRI-linac was 0.993 ± 0.013 (k = 1). The average kB→factor 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 of kB→for 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

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 kQmsrQ0fmsrfref, a correction for beam quality and kB→,Qmsrfmsr, for the impact of the magnetic field on dosimeter response in the magnetic field. Literature based values for kB→,Qmsrfmsr 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

Modelling the x-ray source for the Australian MRI-Linac

MRI-guided radiotherapy allows real-time imaging during treatment however the magnetic field influences the dose distribution in the patient. An accurate model of the radiation beam and the encompassing magnetic field is important to predict dosimetry changes. The purpose of this work is to develop a Monte Carlo model of the Australian MRI-Linac to be used as input into a dose calculation tool for treatment planning. The Australian MRI-Linac is a 1 T inline system with a 6MV flattening filter free photon beam. Commissioning measurements were undertaken both with and without the magnetic field present, PDDs and profiles were used to develop a model with the Geant4 toolkit. To date the model at 0 T matches within ± 2% of measured data. Ongoing work involves measurements at 1 T at various linac to MRI isocentre distances, the magnetic field model at each configuration is also under development

The Australian MRI-Linac Program: measuring profiles and PDD in a horizontal beam

The Australian MRI-Linac consists of a fixed horizontal photon beam combined with a MRI. Commissioning required PDD and profiles measured in a horizontal set-up using a combination of water tank measurements and gafchromic film. To validate the methodology, measurements were performed comparing PDD and profiles measured with the gantry angle set to 0 and 90° on a conventional linac. Results showed agreement to within 2.0% for PDD measured using both film and the water tank at gantry 90° relative to PDD acquired using gantry 0°. Profiles acquired using a water tank at both gantry 0 and 90° showed agreement in FWHM to within 1 mm. The agreement for both PDD and profiles measured at gantry 90° relative to gantry 0° curves indicates that the methodology described can be used to acquire the necessary beam data for horizontal beam lines and in particular, commissioning the Australian MRI-linac

Introducing dynamic dosimaging: Potential applications for MRI-linac

The new era of intra-fraction dose tracking in radiation therapy delivery demands new dosimetry methods, whereby a moving frame of reference as a function of time may be required. This introduces a new paradigm into radiation therapy dose verification. The term we propose to describe this is dynamic dosimaging, which by our definition is tracking the location of a dosimeter array in real time during on-line radiation dose acquisition

Bulletin of experimental biology and medicine : a publ. of the Academy of Medical Sciences of the USSR

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