Reducing the margin of error: MRI-guided radiotherapy

Part four of our blog series on radiotherapy takes a look at how MRI scans are helping to improve the accuracy of radiotherapy and reduce side effects. 

Say it quietly, but there’s a radiotherapy revolution coming.

Doctors are on their way to developing a kinder, more effective approach for delivering radiotherapy. And in the process they hope to spare patients time and side effects, and the NHS cash.

To do this they’re combining imaging techniques that see inside patients, such as MRI, with radiotherapy machines that deliver treatment. In doing so they can pinpoint the tumour with supreme accuracy, and target radiotherapy to exactly where it needs to go.

Where before they had to target a broad area around the tumour to make sure that it was completely covered with radiotherapy, they may now be able to reduce that margin. This means less healthy tissue receives high doses of radiation, potentially reducing side effects.

And with doctors sure that the tumour is precisely receiving the desired radiotherapy dose, they can also explore cutting down the number of doses that need to be given, potentially sparing patients time in hospital.

Clinical trials are underway to test this approach in several types of cancer. And according to those involved, the results could be transformative.

Problem-solver

Professor Kevin Harrington, joint head of the Division of Radiotherapy and Imaging at The Institute of Cancer Research (ICR), London, says that until now, the approach to matching images of tumours to radiotherapy doses has been pretty basic.

“We were taking pictures of the rough location of a tumour, drawing rectangles with a felt-tip pen and irradiating everything from one side of the patient to the other within those areas,” he says.

As a result, lots of normal tissue within those areas would be hit, potentially causing side effects. But gradually this has changed with the introduction of imaging techniques to pinpoint a tumour and guide radiotherapy.

“This usually uses x-rays or mini-CT scans, but the best imaging to see most tumours is magnetic resonance imaging (MRI),” says Dr Alison Tree, a radiotherapy expert at The Royal Marsden Hospital (RMH) in London and the ICR.

The MRI machine gives a crystal-clear look inside the body, including the tumour and the normal tissue and organs around it.

“Usually the patient is scanned and moved to get them and the cancer in exactly the right position – then we have to turn off the imaging to treat the patient,” says Tree. “With MRI-guided radiation we can continue to image and ensure precise positioning all the way through radiotherapy.”

And if the cancer moves outside of the radiotherapy beam the doctors can stop and adjust accordingly.

“We can follow how the tumour moves in real time to tackle cancers that are tough to treat – like lung, pancreatic and gastrointestinal cancer,” Harrington adds. “These tumours move quite a lot, so our margins around them have to be large in order to hit the target.”

These safety margins are added around the tumour to accommodate movement of internal organs and to make sure all the tumour gets hit. But these margins include healthy tissue.

“With MRI-guided radiotherapy we’re able to change or adapt the radiotherapy plan to react to changes in the anatomy of the patient or tumour,” Tree adds. “This increases precision and represents a big step forward in radiotherapy treatments.”

Harrington suggests an analogy to work out how much volume is added to something by adding just a couple of millimetres.

“Peel an orange and stack the peel up next to it,” he says. “The peel can be a similar volume to the orange but it’s only a couple of millimetres thick.

“And that’s what we do when we irradiate patients – we add 3-5 millimetres around the tumour, and we irradiate basically the same volume of normal as tumour tissue.”

So by making those margins narrower, doctors are able to spare normal tissue. This reduces side effects, lets them deliver more radiation to the tumour, and potentially make treatment more effective.

Thinking big

A drive behind MRI-guided radiotherapy is to give fewer and fewer doses of radiation with much greater accuracy.

“Part of the reason we might now deliver 6 weeks of radiation is that we take account of the inherent inaccuracy of radiotherapy,” says Harrington. “If you miss some of the tumour on one day then you get it on the next or the one after that.”

“With MRI-guidance you can be pretty certain that you’re hitting the target in exquisite detail every time you treat the patient.”

And there’s the double bonus that you’re minimising the amount of normal tissue that gets a radiotherapy hit.

I hope in the next 10-20 years we’ll see a complete revolution in the way we deliver radiation.

Professor Kevin Harrington

“You can begin to contemplate that, instead of giving 35 doses of radiation, you drop it to 20 doses.”

Harrington says that some colleagues are even looking to a point where they might be able to cure some cancers with one, two or three doses of radiation.

“In terms of convenience for patients and quality of life that would be a massive gain, and for the healthcare system with huge cost savings and freeing up capacity,” he says.

“The more patients you cure the first time you treat them, the fewer patients will need expensive drugs to treat their metastatic disease.”

But there are always going to be cancers found where the disease is too widespread or aggressive to cure it with radiotherapy. MRI-guided radiotherapy, as with other treatments, won’t be a silver bullet.

But what about now?

The Royal Marsden and the ICR are part of a global collaboration on MRI-guided radiotherapy research. They’ve identified 9 key cancer types that could especially benefit from it – lung, prostate, breast, gynaecological cancers, oesophagus, head and neck, pancreas, brain, and rectal cancers.

This collaboration means that the expertise of radiotherapists, clinicians, physicists, radiographers, engineers and many others across the world can be brought together to maximise treatment now, and also in the future.

Even as the first trials run, the groups will be working out what can be improved for the next phase – something that Harrington is rightly proud of.

“One of the things that’s made the UK very strong in research is that in many other countries they will simply adopt technologies without testing them formally, because they get funded and paid to use the biggest and the best technologies, and patients will hunt after them,” he says.

“In the UK it’s never been that way – we’ve had to demonstrate that treatments work before we can have them, which means doing clinical trials. This is a big advantage for us.”

The teams are writing the clinical plans to treat their first patients with MRI-guided radiation early next year. And the Christie hospital in Manchester will have a similar system shortly after that.

Tree expects that if these clinical trials demonstrate some benefit for the approach, more centres will buy MRI-guided radiotherapy machines.

“It has applicability to nearly all cancers, as practically all tumours are better seen with MRI than CT or x-ray,” she says.

And while he waits on the results from the first trials, Harrington is quietly confident: “I hope in the next 10-20 years we’ll see a complete revolution in the way we deliver radiation.”

Michael