Radiation therapy is an important component of cancer treatment, where a multi-disciplinary approach is the standard of care. Modern computer technology in treatment planning, coupled with continuous innovation and in the improvement in the knowledge of the radiobiology of cancer, has led to the development of new radiation therapy techniques.
Over the past 30 years, we have moved from two-dimensional to three-dimensional radiation therapy with the development of inverse computational algorithms allowing the planning of precise, complex radiation delivery patterns.
Modern techniques such as image-guided intensity-modulated radiation therapy (IGRT) and stereotactic ablative radiation therapy (SABR) have been implemented, which demonstrate improved shaping of the radiation dose to the target and away from surrounding healthy organs, allowing for radiation dose escalation to tumours with decreased toxicities.
Globally, we are witnessing the implementation of the latest revolution in radiation therapy, the magnetic resonance imaging linear accelerator (MR Linac), a technology that has integrated a diagnostic quality 1.5T MRI scanner with the treatment delivery linear accelerator. The MR Linac can provide real-time images during treatment without increased radiation exposure to deliver what is referred to as online MR-guided adaptive radiation therapy (MRgRT).
Benefits and clinical indications
MRgRT represents a paradigm shift in radiation therapy. With enhanced visualisation of internal organs using the excellent soft tissue delineation on the MRI and the ability to monitor anatomical structures, position and physiological motions during treatment delivery, we can safely escalate the radiation dose and reduce margins during treatment planning.
Readily available imaging at every treatment session allows for adaptive radiation therapy by which the dose delivered at each session can be evaluated and optimised on the go, taking into account daily variations in patient position and internal organ physiological motions to mitigate toxicities.
The versatility of the MR Linac makes it possible to help treat numerous tumour sites where the soft tissue contrast using MRgRT will allow safe high-precision treatment with optimal sparing of surrounding healthy organs.
Most experienced centres and the European Society for Radiotherapy and Oncology (ESTRO) have recommended clinical application to prostate cancer, brain tumours, pancreatic cancer, liver cancer and pelvic lymph nodes. Moving intra-abdominal or pelvis targets would specifically benefit from the MRgRT motion management approach due to the deep-seated nature, the difficulty of placing fiducials markers and the tendency for variable deformation between treatment sessions.
Another noteworthy advantage of the MR Linac is ‘radiomics’, which is the application of quantitative imaging analysis to predict tumour response or resistance based on functional, structural, or physiological MR imaging data in addition to anatomical images obtained by the MRI. Radiomics can be integrated with traditional clinical data to establish tumours that will respond to treatment and assist with decision-making to identify tumours that would benefit from radiation dose escalation.
There are four MR Linacs in Australia, the latest of which is in Murdoch, Western Australia. It is integral to provide specific training and credentials for staff to ensure the safe and effective utilisation of the MR Linac, and all centres have a rigorous training and credentialing process in place.
Challenges
Despite the superior imaging quality of the MR Linac, the visualisation of targets may be affected by breathing-induced motion artefacts. To address this, a compression belt is recommended to reduce breathing artefacts when treating targets in the upper abdomen.
Furthermore, larger tumours may pose a challenge to treatment due to the limited field of view in the superior-inferior direction on the MR Linac. By nature of MRgRT being fully adaptive, the treatment times are longer, hence, compliance and comfort could be a potential issue for a subset of patients.
Treatment contra-indications include patients with MRI-incompatible implanted materials or claustrophobia.
Finally, higher capital costs, resources, and the complexity of the treatment have constrained the widespread adoption of this technology.
While MR Linac offers a promising solution to high-precision treatment with more outcome data now being published, the long-term outcomes of MRgRT remain unclear. To produce real-world evidence, prospective registries have been established, such as MOMENTUM, a multi-institutional, international registry collecting clinical, technical, and patient-reported outcome data. These will pave the way to ongoing evidence-based development of MRgRT to confirm the most suitable treatment sites and techniques to enhance toxicity reductions.
Ultimately, the MR Linac has unveiled a transformative shift in the radiation therapy treatment paradigm. It presents opportunities for greater personalisation of treatment and holds the potential to revolutionise the field of radiation oncology in the coming decades, inspiring hope for improved patient outcomes.
Key messages
- MR Linac offers real-time tumour tracking and delivery, allowing clinicians to target tumours more precisely
- The integration of MRI with a linear accelerator provides superior soft-tissue imaging to aid accurate delineation of the tumour and surrounding healthy organs
- While MR Linac is a promising technology, high capital cost and complexity limit widespread adoption. Ongoing research and advancements will likely improve future accessibility.
– References available on request
Author competing interests – nil