MRI can help to categorize tissues as malignant or non-malignant both anatomically and functionally, with a high level of spatial and temporal resolution. This non-invasive imaging modality has been integrated with radiotherapy in devices that can differentially target the most aggressive and resistant regions of tumours. The past decade has seen the clinical deployment of treatment devices that combine imaging with targeted irradiation, making the aspiration of integrated MRI-guided radiotherapy (MRIgRT) a reality. The two main clinical drivers for the adoption of MRIgRT are the ability to image anatomical changes that occur before and during treatment in order to adapt the treatment approach, and to image and target the biological features of each tumour. Using motion management and biological targeting, the radiation dose delivered to the tumour can be adjusted during treatment to improve the probability of tumour control, while simultaneously reducing the radiation delivered to non-malignant tissues, thereby reducing the risk of treatment-related toxicities. The benefits of this approach are expected to increase survival and quality of life. In this Review, we describe the current state of MRIgRT, and the opportunities and challenges of this new radiotherapy approach.
Radiotherapy approaches must balance the delivery of a therapeutically effective dose of radiation to target tissues whilst minimizing damage to the surrounding non-malignant tissue; however, anatomy and physiology are dynamic, making it challenging to accurately target tumours during radiotherapy.
MRI-guided radiotherapy (MRIgRT) uses MRI during the delivery of radiation, thus enabling more-precise cancer targeting and avoiding irradiating surrounding non-malignant tissues.
MRIgRT enables the differential targeting of tumour regions with higher doses of radiation than can be delivered using traditional approaches, providing new options to treat the most aggressive and resistant regions of the tumour.
MRI-guided linear accelerator systems, which are MRIgRT devices that integrate an MRI scanner and a linear accelerator, are rapidly being deployed in the clinic, with opportunities for outcome improvements and the challenges of added complexity and cost.
Rapid clinical utilization of MRIgRT is currently limited by the need for increased infrastructure, treatment and staffing costs, and the paucity of medium-term to long-term clinical data.
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P.J.K. acknowledges funding from the Australian Government National Health and Medical Research Council (NHMRC) Investigator (1194004) and Program (1132471) grant schemes. P.Z.Y.L. receives support from the Cancer Institute of New South Wales (NSW) (fellowship ECF/1032). A.C.T. acknowledges research funding from Cancer Research UK (grants C7224/A28724 and C33589/A28284). D.E.J.W. acknowledges support from the Cancer Institute of NSW (fellowship ECF/1015). B.W. acknowledges support from the NHMRC Early Career Fellowship scheme (1163010). A.C.T.’s involvement is supported by the UK National Institute for Health Research (NIHR) Biomedical Research Centre at The Royal Marsden NHS Foundation Trust and the Institute of Cancer Research, London. The views expressed are those of the authors and not necessarily those of the NIHR or the UK Department of Health and Social Care (A.C.T.). All authors thank H. Ball and J. Johnson (both at the University of Sydney) for help with manuscript and artwork preparation, respectively.
P.J.K. is an inventor on two patents relating to MRI–linac systems: US#8,331,531 and US#9,099,271. A.C.T. receives research funding from Accuray, Elekta and Varian, and honoraria and travel assistance from Elekta. U.A.v.d.H. receives research funding from Elekta and Philips Healthcare. The other authors declare no competing interests.
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Widely used medical imaging procedure that measures magnetic properties of tissue. Diseased tissues, such as cancer, often have different magnetic properties compared with surrounding healthy tissues, enabling them to be precisely located. The location of the diseased tissue within the body is used for targeting disease with radiation beams during radiotherapy.
- MRI-guided radiotherapy
(MRIgRT). Process of exploiting the soft-tissue imaging capacity and flexibility of MRI for targeting areas of pathology, predominantly cancer, with radiation. Although MRI scanners have had a long standing role in radiation therapy for pretreatment imaging of cancer and to measure treatment response, in the context of this Review, MRIgRT refers to the rapidly growing application of MRI guidance whilst a patient is receiving radiotherapy. This functionality is enabled by use of an MRI-guided linear accelerator.
- MRI-guided linear accelerator
(MRI–linac). MRIgRT device that integrates an MRI scanner and linear accelerator. Most standard (non-MRI) linear accelerators used for cancer treatment have X-ray imaging systems to combine imaging with the delivery of radiation.
Portmanteau term for linear accelerator, a device that accelerates charged particles to create X-rays and deliver radiotherapy.
The part of the linear accelerator that the radiation beam is mounted on, which rotates 360 degrees. This rotation allows many possible angles of the radiation beam to enter the patient. Typically, 5–11 beam angles are used in each patient treatment.
- Tumour tracking
Imaging the tumour in real time to enable improved radiation beam tumour targeting by either gating the radiation beam or having the radiation beam continuously follow the moving tumour by adjusting the multi-leaf collimator beam shaping device.
- Beam gating
Turning the radiation beam on when the tumour moves within a predefined treatment boundary and turning the beam off when the tumour moves outside the treatment boundary. Beam gating protects non-malignant tissues from irradiation if a target moves from its expected treatment position.
(OARs). Non-malignant structures within the human body located sufficiently close to a tumour to be at risk of damage from the radiation delivered during radiotherapy.
- Geometric fidelity
The degree of exactness with which structures within reconstructed images replicate the size, shape and location of patient anatomy. MRIgRT demands a high level of geometric fidelity to ensure that radiation beams are accurately targeted.
The part of the linear accelerator that shapes the radiation beam to conform to the tumour target.
Physical devices used to assist measurements of image quality, dose delivery and other scientific investigations for MRIgRT.
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Keall, P.J., Brighi, C., Glide-Hurst, C. et al. Integrated MRI-guided radiotherapy — opportunities and challenges. Nat Rev Clin Oncol 19, 458–470 (2022). https://doi.org/10.1038/s41571-022-00631-3
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