November 25, 2018 8:00 am

A simple blood test could someday detect cancer. But there’s a catch

WATCH: What is a "liquid biopsy" and what can it be used for?

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One day, people might be able to get a cancer test as easily as having their cholesterol checked.

“Liquid biopsy” – a blood test – is an active area of research for cancer diagnosis. While the research is still in early stages and the test is years, maybe more than a decade, away from being available to the general public, experts say it could be valuable.

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“I think for early detection for cancers that are deadly and potentially curable in the early stage, this is potentially going to be a game-changer,” said Dr. John Lewis, of the department of oncology at the University of Alberta.

How a liquid biopsy test works

The test works by detecting changes in the bloodstream, such as fragments of mutated DNA or particular molecular changes that indicate the presence of cancer. Right now, cancers are diagnosed through tissue sample biopsies, a much more invasive method.

Liquid biopsies can be used for a number of purposes, like monitoring whether a cancer treatment is actually shrinking the tumour, for personalizing treatments based on the particular gene mutation that a patient has, as well as for detection. At least, that’s what researchers hope.

For a recent study published in Nature, a Toronto-based research team was able to train a computer program to recognize some chemical changes in the bloodstream for people with early-stage cancers, and even identify what kind of cancer the patient had, from a list of a few different types.

READ MORE: Cancer blood test that finds 8 kinds of tumours sounds ‘promising’

“This opens the door, it’s still in the research stage, but it opens the door of liquid biopsy for early cancer detection,” said study co-author Daniel De Carvalho, senior scientist at Princess Margaret Cancer Centre.

Another study, published in Science in early 2018, also showed promising results for a cancer-detecting blood test, which was able to identify eight different cancers with a median success rate of 70 per cent in more than 1,000 patients.

These procedures still need to be improved, refined and tested before they make it to the mass market, said De Carvalho, and that will take years. But he thinks that identifying cancer early could make a big difference for some people.

“The earlier we identify the cancer, the easier it is to treat the patient.”

For these tests, “The goal is to find it while the tumour is so small that you can take it out,” said Rayjean Hung, a Canada research chair in integrative molecular epidemiology in the Sinai Health System in Toronto.

READ MORE: These two cancers aren’t detected early enough, and it’s costing Canadian lives

Hung, who is another co-author of the Nature study, thinks liquid biopsies could be particularly helpful for patients with lung, pancreatic or other aggressive, fast-growing cancers where early detection makes a big difference in treatment outcomes.

Treating the cancer

Treatment is still a big question though. What do you do if you find cancer, but can’t do anything about it?

“It’s a very active discussion right now,” Lewis said.

“If we know something is there and we don’t have a clear, clinical path forward, is that information useful?”

In prostate cancer, for example, PSA (prostate-specific antigen) tests have been criticized for leading to overdiagnosis.

“The PSA test, for instance, has allowed a lot of men to test their prostate cancer earlier, but the side-effect of PSA screening has been that a lot of men with non-aggressive indolent prostate cancers have also been diagnosed,” he said. This test has been criticized for leading to unnecessary additional testing or even treatments for something that may never have caused a problem.

“With the diagnosis of cancer, there’s anxiety and burden based on the diagnosis, and typically, most people want to do something about it. Often, there is not a clear clinical benefit or even an option to treat.”

This is less of a problem in aggressive forms of cancer, like lung cancer, said Hung. “We rarely see any lung or pancreatic cancer that sits there. So for those ones, overdiagnosis becomes less of an issue. But it is an issue for the other types.”

These tests would also be unlikely to lead directly to a cancer diagnosis, she said. Instead, patients would probably have to go for CT scans and other imaging tests following a positive result.

READ MORE: Could this new blood test detect and locate cancer early on?

De Carvalho hopes that as his computer program trains itself to better recognize different cancers, it will be able to know which ones won’t lead to serious medical issues and which will, and identify them accordingly.

For less aggressive cancers like prostate cancer, the important thing is to identify which cancers will kill the patient, not whether the patient has cancer, said Lewis.

“So liquid biopsy and the whole field are going to have to be more specific than, ‘Do I have cancer?’ It’s going to have to be specific about, ‘Do I have an aggressive form of cancer that I need to act upon?’”

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