Magnetic resonance guided radiotherapy (MRgRT) shows great potential for the further improvement of cancer treatment. It offers increased accuracy in defining the target and organs at risk as well as real-time imaging for online verification of dose delivery, online adaptation and treatment optimisation.
However, MRgRT requires measurement of dose and dose distributions in the presence of a magnetic field, which currently decreases dosimetrical accuracy considerably. In MRgRT, the role of CT images in conventional radiotherapy will be replaced by MRI, which poses other and higher demands on the geometrical accuracy of MR images. Both these factors are still holding back the wide-scale clinical implementation of this promising technique.
To allow for the widespread clinical implementation of MR guided radiotherapy for cancer treatment, capabilities to measure both the influence of the magnetic field on the detectors and the radiation beam dose distribution must be developed. Geometrical accuracy of the images used in treatment planning is essential in delivering dose distributions to the patient as intended.
In this Joint Research Project experts and practitioners from various disciplines in medical physics and metrology are working together on the metrological framework and methodologies for accurate dose delivery and MR imaging.
The evolution of MRgRT has only just started in the context of academic centres. The impact of this project on this evolution the coming years and on European healthcare in general will be high. More specifically, it will impact the following developments:
- A harmonised metrological framework will be in place for quality assurance to enhance consistency in multi-centre clinical trials organised to investigate the clinical benefits of MRgRT.
- Ultimately, 0.7 – 1.2 million European cancer patients annually will have access to MRgRT and will potentially benefit from increased life expectancy and enhanced quality of life.
- MRgRT is a potential non-invasive treatment for a range of cancers, for which conventional radiotherapy, to date, has proved inadequate due to organ movements. The eligibility of these tumour sites for MRgRT will be investigated.
- The competitiveness of European manufacturers of MRgRT facilities and detector equipment will increase.
Metrological and scientific impact
On a metrological and scientific level, this project will lead to:
- Traceability of clinical reference dosimetry for MRgRT to primary standards.
- Fundamental dosimetrical concepts to validate MC codes for radiation transport in the presence of magnetic fields.
- Novel concepts of QA using MR compatible phantoms and methods to determine treatment safety margins for MR based dose delivery.
The ultimate goal of this Joint Research Project is the safe clinical implementation and support of future innovations in MR guided radiotherapy (MRgRT) by developing the metrological capacity in dosimetry and imaging.
The specific scientific and technical objectives of this project are:
- To develop a metrological framework of primary and secondary standards for traceable dosimetry under reference conditions for MR guided radiotherapy, which will form the basis of future dosimetry protocols (CoPs). This will include determining input data and establishing a formalism for reference dosimetry.
- To develop methodologies for measuring treatment planning system (TPS) input data for MR guided radiotherapy. This should include the determination of detector characteristics for commercially available detector systems and secondary standards in hybrid fields. It also includes characterisation of the radiation field based on measurements and Monte Carlo modelling.
- To develop methodologies to assess the accuracy of the Monte Carlo based radiation transport algorithms in external magnetic fields.
- To evaluate MR based dose delivery under static and dynamic conditions.
- To facilitate the uptake up of recommendations for dosimetry and MR related quality assurance of MR guided radiotherapy developed in the project by clinicians and industry in order to enable hospitals to perform quality assurance based on traceable measurements and support improvements for dosimetry in MRgRT.
To achieve the goals, the following work packages have been determined:
In external beam radiation therapy the ability to measure a precisely absorbed dose under well-defined reference conditions (reference dosimetry) is an essential prerequisite for beam calibration. For MR guided radiotherapy (MRgRT), primary standards, which allow absorbed dose measurements under the influence of magnetic fields, are still lacking. Consequently, there is no consistent traceability route and there are no Codes of Practices that can be applied by hospitals for reference dosimetry in the presence of magnetic fields.
In the framework of the EMRP funded project JRP HLT06 MRI Safety, a water calorimeter was developed and its feasibility for performing dose measurements in the presence of a magnetic field was shown. The aim of work package 1 is to develop a metrological framework (primary and secondary standards) for traceable dosimetry under reference conditions for MR guided radiotherapy. For this purpose, the water calorimeter will be optimised and used for absorbed dose measurements with a target uncertainty of 1.0 % (k = 2) in the presence of magnetic fields at integrated radiotherapy MRI facilities. This will provide a metrological basis (primary standard) for subsequent investigations.
Furthermore, correction factors to account for the change of the response of ionisation chambers in high-energy photon beams in the presence of magnetic fields will be determined by calibrations traceable to the water calorimeter and other indirect methods based on secondary standards. To support a broad range of MRgRT modalities (currently commercially available or under development), these correction factors will be measured dependent upon
1. the energy of the beam,
2. the strength of the magnetic field,
3. the orientation of the detector in the magnetic field.
Finally, a formalism will be developed and validated that allows absorbed dose measurements in high-energy photon beams in the presence of magnetic fields with a target uncertainty of 2.0 % (k = 2).
Dose calculation algorithms in commercial Treatment Planning Systems (TPS) rely heavily on the accuracy of the underlying beam model and of the measured dosimetrical input data that is fed into this beam model. Furthermore, in the commissioning and acceptance phase of new radiotherapy equipment, hospitals need to perform measurements to assess the accuracy of the dose algorithm in well-defined radiation fields. In addition, quality assurance (QA) of the linac facility is essential to ensure accurate dose delivery to the patient and to minimise the possibility of accidental exposure.
The overall aim of work package 2 is to develop methodologies for the accurate measurement of TPS input for MR guided radiotherapy (MRgRT) as well as for machine measurements for QA for wich both the geometrical and the dosimetrical uncertainties are relevant. Target uncertainties for both vary among the different types of measurement (Percentage Depth Dose (PDD), penumbra, etc.) and should at least be in line with the acceptability criteria for the determination of radiation beam characteristics, and TPS dose calculations published for conventional radiotherapy.
This includes the characterisation of the suitability of commercially available point detector systems for the measurement of input data in the presence of the magnetic field and the characterisation of 2D and 3D dose mapping systems for machine QA measurements. An essential aspect is input from WP3 based on highly accurate Monte Carlo calculations of dose distributions and detector response in combination with the measurements performed in WP2. A selection of detector systems for the measurements will be drafted on the basis of these findings.
The Monte Carlo method is an integral part of existing radiotherapy reference dosimetry. This technique not only allows the characterisation of the physical properties of beams and radiation detectors with high accuracy, it also serves as a reference method for a variety of situations such as providing valuable data for clinical dosimetry. In the context of MR guided radiotherapy (MRgRT), recent studies have shown significant effects on absorbed dose in the presence of magnetic fields (B-fields). Monte Carlo is expected to play a key role in calibrating MRgRT machines.
The aim of this work package is to develop methodologies for testing and validating Monte Carlo based radiation transport algorithms for accurate beam modelling and detector response simulations in external magnetic fields. General-purpose codes integrating B-fields are yet to be properly tested in calculations relevant to reference dosimetry for both self-consistency and experimental benchmarks. The benchmarked Monte Carlo codes for application in magnetic fields will then be used to model MRgRT facilities using a method previously applied for conventional radiotherapy facilities. The results of these beam simulations will be used as input for WP2. Furthermore, the benchmarked Monte Carlo codes will be used to calculate correction factors (kB) for a set of reference ionisation chambers and other detectors, which will be used as input for WP1 and WP2.
The clinical implementation of MR guided therapy (MRgRT) with dedicated MR-integrated radiation devices requires thorough quality assurance (QA) measures (general, patient-specific and workflow) to guarantee that the dose distribution is delivered as intended in treatment planning. Inter- and intra-fractional motion, such as breathing, must be detected and compensated for within adaptive treatment strategies.
When compared to conventional radiotherapy, the technical realisation of the workflow in MRgRT poses significant challenges due to different image contrast, potential magnetic field distortions and interactions of the magnetic field with the radiation field and the dosimetric equipment. Therefore, specific QA tests are required, which include the employment of phantoms and measurements, as well as simulations to estimate the residual uncertainty.
The aim of this work package is to evaluate MR based dose deposition under static and dynamic conditions. This will facilitate the clinical implementation of MRgRT with the development of general-, patient-specific and workflow-QA-tests. General QA procedures include methods to assure the accuracy of the dose delivery and imaging accuracy of the treatment machine. Patient-specific QA procedures aim to prevent malfunctioning of components for the individual patient. Although these measures may assure the accuracy of each single step, end-to-end tests are also necessary to validate the accuracy of the intended workflow. Within these workflow-QA procedures, end-to-end tests in the presence of inter-fraction and intra-fraction organ motion are of special importance.
This working package includes the development of tests to measure and assess, by means of simulations, the overall geometrical as well as dosimetric uncertainties in 2D and 3D within the framework of time-adaptive radiotherapy. All measurement and simulation procedures developed in this work package will be vendor-independent with respect to the MR-integrated irradiation device.
The aim of this work package is to ensure the wide dissemination of the knowledge generated within this project. It is also important to gather information and on-going feedback regarding the needs of the end users and other stakeholders both in terms of those providing input to the project and the European end-user community. This work package will also ensure that the developments achieved in this project will be available for exploitation by a wide range of stakeholders in industry, the metrology and scientific community and societal institutes and bodies.
A stakeholder committee will be regularly informed about the results of the project. Also, knowledge will be shared by writing various papers for journals and presenting papers at conferences. Updates will also be shared on this website. Moreover, trainings will be organised on:
reference dosimetry for MR guided radiotherapy;
Monte Carlo radiation transport in the presence of magnetic fields;
the simulation platform developed in WP4 for assessing geometrical and dosimetrical uncertainties relevant for MR guided radiotherapy.
Finally, the developed simulation platform for the assessment of geometrical and dosimetrical uncertainties, developed in WP4, will made freely available on the project’s website.
The project was finalised in 2019.