Therapy radiographers preparing the patient for treatment.
In the fifth instalment of our blog series on radiotherapy, we find out about a technique called SABR. We cover what it is, who it works best for and the kind of trials that are using it.
There are lots of different types of cancer and they develop in lots of different places, and so there are many ways of having radiotherapy treatment.
Dr Louise Murray, from the University of Leeds, focuses on a particular technique called SABR (stereotactic ablative body radiotherapy). SABR may sound like the weapon of a Jedi Knight, but instead of transmitting beams of light, SABR transmits intense, focused beams of radiation.
Unlike conventional radiotherapy treatments that give patients small amounts of radiation each day over a few weeks, SABR is quick and to the point.
“SABR treatment uses a very high dose of radiation per treatment but only a small number of times,” says Murray.
Precision is key
Because SABR beams are so intense, they must be extremely precise.
“To make sure the dose of radiation is given accurately, we need to image at each treatment,” says Murray. “When we treat a tumour we always do imaging to show that we’re hitting the right spot.”
Hitting a wider patch, like conventional radiotherapy does, with this much radiation would be very damaging to healthy tissues, so the shape of the beam in SABR is adapted to fit the tumour closely.
“It’s a very precise art; the margin around the tissue is very small to reduce the number of healthy cells hit,” says Murray.
Conventional radiotherapy is useful because it uses a wider beam that reaches out to the area around the tumour. It hits stray cancer cells that may have moved further away, such as in more developed cancers.
But the downside to this is that healthy cells are likely to be among the stray cancer cells, and will be affected by the radiation too. As a compromise, each conventional radiotherapy treatment session uses a lower dose to reduce the likelihood of healthy cell damage, but this means more treatment sessions are needed, which results in more hospital visits for patients.
And even with a smaller dose per treatment, conventional radiotherapy can still come with side effects.
But SABR is designed to avoid these issues.
SABR’s stronger beam means tumours can get a very large radiation hit in fewer visits. This allows a patient to finish treatment with SABR quicker – sometimes within a week, and usually in two.
And because SABR gives such a large dose of radiation directly to the tumour, there’s a greater chance the tumour cells will die, making it a very effective treatment option.
Margin of error
Because the treatment area for SABR is so much smaller and the margins around the tumour are narrower, there’s much less chance it will hit healthy cells, so long-term side effects aren’t normally an issue for most patients.
“We can have good control over the tumours we’ve treated,” says Murray, “but that doesn’t necessarily stop other tumours popping up at a later stage in other places.”
And this is a downside of SABR. As it’s so precise and doesn’t venture much further than the boundary of the tumour, it’s limited in the types of cancer it can treat.
“As a general rule we use it on small cancers,” says Murray. These also need to be in the right place.
“Tumours have to be in an area where there are no important organs near them, like the heart or major nerves,” she says. “So predominantly we use it to treat peripheral cancers on the lung, but it can also be used to treat tumours in other areas, including the liver, spine and prostate.”
For a small cancerous nodule on the outside of an organ, like on the edge of the lung, SABR is perfect for destroying tiny tumours before they get out of hand.
Professor Corinne Faivre-Finn, a Cancer Research UK-funded radiotherapy expert at the Christie Hospital in Manchester, says SABR is also a great option for older patients who have other medical conditions to worry about on top of a cancer diagnosis.
“For lung cancer SABR is extremely convenient,” she says. “Patients over the age of 70 are suitable for SABR.”
More to explore
“At the moment, SABR is the standard of care for only one group of patients. It’s given to people who are not eligible for surgery and have a type of cancer called non-small cell lung cancer that hasn’t spread,” says Faivre-Finn.
Clinical trials in lung cancer patients have been challenging. “There were a few trials that tried to get going but they all failed to recruit patients,” she adds.
“The majority of people seem to have set views that they prefer surgery. The thinking at the moment is, if you’re fit for surgery, then people want the cancer out of them,” says Faivre-Finn.
This could be why research comparing SABR to other treatment has fallen behind.
Murray says the major step now is to prove SABR works by putting it head-to-head against standard treatments like conventional radiotherapy and surgery.
That’s why we’re supporting the CORE trial that looks into using SABR for breast, prostate and non-small cell lung cancer that has spread.
As well as curing some cancers, experts think it could also have a future in palliative care. International studies are underway to see if SABR can improve quality of life in patients for whom other treatment has failed.
There’s also work going on to see if SABR is better than surgery and standard radiotherapy for prostate cancer in, for example, the PACE trial.
PACE is also looking at ways to improve SABR’s accuracy.
“A lung cancer for example is easy to see on a scan,” says Murray. “It’s usually a white blob on black, but a liver cancer or prostate cancer is more grey on grey, so there’s the option to insert metallic markers to improve the image guidance.”
This potentially reduces the treatment margin added around the tumour and helps radiographers know they’re hitting the right place.
Improving imaging in SABR treatment should make it even more precise, and could even make it fit for use in a wider range of cancer types.
Though SABR may not be for everyone right now, for those who have it, it does its job well.
And with further testing both Murray and Faivre-Finn agree that it will only get smarter, faster and even more precise.