Can a blood test tell patients if their cancer is coming back? Researchers are on it

Researchers at Princess Margaret Cancer Centre in Toronto are working on a large trial to determine if a blood test can detect tiny amounts of cancer remaining after patients have been treated.  

Lead investigator Dr. Lillian Siu says smaller studies around the world have shown evidence that cancer DNA can show up in the blood in amounts too small to be identified by CT scans. 

But large-scale studies are needed to prove that so Siu and her team are enrolling 7,000 patients who have completed their radiation, chemotherapy or other cancer treatments and testing their blood to find microscopic quantities of tumour DNA.   

If the test — also known as a liquid biopsy — is positive, those patients could get additional experimental treatments, such as new immunotherapies, to try to prevent the cancer from coming back. 

If the test is negative, Siu said it could show that the cancer is really gone and further sessions of chemotherapy or radiation could be stopped to minimize side-effects from unnecessary treatments.

Siu says the trial — called SHERLOCK — also aims to find out if the effectiveness of the blood test varies between different cancer types.

Clinicians and researchers have been looking at liquid biopsies to find tiny remnants of cancer left over after treatment — known as molecular residual disease — for the past decade, she said. 

After collecting blood samples and matching them to whether or not cancer actually returned in patients, there is now a “substantial amount of data to show that people who have positive molecular residual cancer, their cancer has a very high chance of returning,” said Siu, who is the scientific lead at the hospital’s Peter Gilgan Centre for Early Cancer Detection Research.

But she emphasized that although promising, blood tests to predict cancer recurrence are “not standard of care” at this point and SHERLOCK and other clinical trials need to be completed. 

The researchers plan to follow the patients for at least five years, Siu said. 

“You need to have the long-term followup to know whether the test is actually predicting longer-term outcome(s), so just stopping at one year is not going to be sufficient in terms of monitoring.”

Siu hopes SHERLOCK will help build the evidence needed to ease one of the biggest fears of cancer patients: that their cancer will come back.  

“Most patients, even after curative treatment, whenever they come back to the clinic for a followup, I can see that they have fear in their eyes,” she said.  

Many breathe a sigh of relief when their CT scans come back clear, she said, but then the anxiety happens all over again at their next scan.  

“(They wonder) when am I actually free from having this fear that the cancer will come back?” 

Gillian Vandekerkhove, an assistant professor at the University of British Columbia who studies bladder cancer and liquid biopsies, said a lot of research in the field has focused on specific types of cancers. 

She welcomed the SHERLOCK trial’s broad focus on multiple types. 

“They’re going to provide a wealth of information and biobank samples that researchers can continue to explore,” said Vandekerkhove, who is not involved in the study.

“Being a Canadian-led initiative is really great for Canadian researchers.”

But she also noted the clinical trial has limitations. 

“I think it’s important to recognize this is an observational study. It’s going to help us understand the technology better and the best-use cases, but there will need to be additional trials. This is not something we’re ready to move into the clinic.”

But Paul Lonergan, a 68-year-old Toronto man who was diagnosed with throat cancer about three years ago, believes the research has already paid off for him. 

An avid hockey player, Lonergan said his family physician initially thought he had a virus — but eventually he started “coughing up blood on the ice.”

After seeing an ear, nose and throat specialist, he was referred to Princess Margaret Cancer Centre and went through radiation and chemotherapy. 

Lonergan was also enrolled in a different clinical trial, called the MERIDIAN study, which looked for residual cancer in the blood of patients who had been treated for head and neck cancers. 

“The doctor said, ‘I’ve got good news and a little bit of bad news. The tumour’s gone. There’s fragments of cancer in your blood, which is the bad news, but we have a trial drug that can probably help you,'” he said. 

Lonergan was treated for several more months with a new immunotherapy drug as part of the study. 

“Sure as heck it worked,” he said.

“I’ve done three six-month checkups and I just finished my third one and they said I’m good.”

Lonergan is still having trouble swallowing and drinks smoothies instead of eating solid food — but he’s been able to get back on the ice as he slowly recovers.  

“I don’t care now about being one of the better players or not. I just go out and have fun and exercise and it’s just good to be out and doing that.”

The SHERLOCK study is funded by a $50-million donation from the Peter Gilgan Foundation. 

This report by The Canadian Press was first published July 8, 2026.

Canadian Press health coverage receives support through a partnership with the Canadian Medical Association. CP is solely responsible for this content.

By Nicole Ireland | Copyright 2026, The Canadian Press. All rights reserved.