How is this different from conventional image guidance?
Modern radiotherapy machines commonly use X-ray or cone-beam CT imaging to check position before treatment. These methods work well for many cancers.
MRI can show some soft tissues more clearly than X-ray-based imaging. This can make it easier to distinguish a target from nearby organs in selected parts of the body. An MR-Linac can also acquire images during treatment without adding an imaging radiation dose.
The better image depends on the clinical question. MRI is not best for every structure, and conventional image-guided radiotherapy remains an effective and appropriate approach for many treatments.
What does adaptive radiotherapy mean?
Internal anatomy can change from one day to another. The bladder or bowel may fill differently, the stomach can change shape, and a tumour or nearby organs may move.
With online adaptive radiotherapy, the team obtains an image while the patient is on the treatment couch. They can adjust the outlines and create or select a plan that fits the anatomy of that day. The plan is checked before it is delivered.
Adaptation is more than moving the patient into a better position. It changes the treatment plan to reflect a meaningful anatomical difference. Not every MR-guided treatment needs a new plan every day.
When may MR guidance be useful?
Possible uses include selected tumours where:
- the target or nearby organs are easier to see with MRI;
- soft-tissue anatomy changes between fractions;
- the target moves with breathing;
- a high dose is being delivered close to a sensitive structure;
- adapting the plan could preserve target coverage or reduce dose to normal tissue.
Examples may include selected prostate, pancreatic, liver, adrenal and other abdominal or pelvic treatments. This does not mean that every patient with one of these cancers should have MR-guided treatment.
Can an MR-Linac account for movement?
Some systems can obtain images repeatedly or continuously during delivery. Depending on the equipment and treatment, this may help the team monitor movement, use breath-hold, or pause the beam if the target moves outside an accepted position.
This is one of several ways to manage motion. A 4D CT, breath-hold, gating or another image-guided technique may be more suitable. You can read about those options in How do we target tumours that move during radiotherapy?.
Is MR-guided radiotherapy the same as SABR?
No. MR guidance describes how the treatment is imaged and guided. SABR describes a highly focused dose delivered in a small number of fractions.
SABR can be delivered without an MR-Linac, and an MR-Linac can deliver treatments that are not SABR. The two may be used together when that combination is appropriate.
What are the limitations?
An adaptive MR-Linac appointment can take longer than a conventional radiotherapy appointment because imaging, recontouring, replanning and checks happen while the patient remains on the couch.
The patient must be suitable for MRI and able to maintain the required position. Some implanted devices, metal objects or severe claustrophobia may affect suitability. The machine also has technical limits, and the target needs to be visible well enough for the intended workflow.
MR-Linac treatment is available in a limited number of centres. Its availability is not evidence that it will improve the outcome for every cancer. The clinical benefit of using it should be considered against a well-delivered conventional plan and other established options.
Questions to ask
- What problem would MR guidance solve in my treatment?
- Would the plan be adapted each day?
- How long is each appointment likely to take?
- Will I need to hold my breath or remain still for long periods?
- Is a conventional image-guided treatment an equally appropriate option?
MR-guided radiotherapy is an additional way to plan and guide radiotherapy. The important question is not whether the machine is newer, but whether its imaging or adaptive workflow offers a meaningful advantage for the treatment being considered.
