For many people in Simcoe County who've gone through cancer treatment, the follow-up appointments in Toronto are part of the routine. A new trial out of one of the city's biggest cancer hospitals could eventually change what happens at those appointments, and how much waiting and worrying comes with them.
Researchers at Princess Margaret Cancer Centre are running a large-scale trial to find out whether a simple blood test can catch tiny traces of cancer left behind after treatment, before they'd ever show up on a CT scan.
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What the SHERLOCK trial is testing
Lead investigator Dr. Lillian Siu says smaller studies around the world have already shown that cancer DNA can turn up in the blood in amounts too small for a scan to detect.
Now her team needs proof at scale. They're enrolling 7,000 patients who have finished radiation, chemotherapy or other cancer treatments, and testing their blood for microscopic amounts of tumour DNA.
The test is sometimes called a liquid biopsy.
If it comes back positive, those patients could be offered additional experimental treatments, like new immunotherapies, aimed at stopping the cancer before it has a chance to return.
If it comes back negative, Siu said it could mean the cancer is genuinely gone, which would let doctors stop further rounds of chemo or radiation and spare patients the side effects of treatment they don't actually need.
The trial, called SHERLOCK, is also trying to figure out whether the blood test works equally well across different types of cancer.
A decade of research building toward this
Clinicians have been studying liquid biopsies to detect leftover cancer, known as molecular residual disease, for about ten years, Siu said.
After matching blood samples against whether cancer actually came back in patients, she said there's 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."
Siu is the scientific lead at the hospital's Peter Gilgan Centre for Early Cancer Detection Research.
Still, she was clear that the science isn't there yet for everyday use. Blood tests to predict cancer recurrence are "not standard of care" right now, and trials like SHERLOCK still need to run their course.
Researchers plan to follow patients for at least five years.
"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 said.
The fear cancer patients carry
Siu hopes the trial will help ease one of the most common fears among cancer patients: that the disease 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.
She said many patients feel a wave of relief when a CT scan comes back clear, only for the anxiety to build again ahead of the next one.
"(They wonder) when am I actually free from having this fear that the cancer will come back?"
An outside researcher's take
Gillian Vandekerkhove, an assistant professor at the University of British Columbia who studies bladder cancer and liquid biopsies, said much of the existing research in this field has focused narrowly on specific cancer types.
She welcomed SHERLOCK's broader approach, which spans multiple cancers at once.
"They're going to provide a wealth of information and biobank samples that researchers can continue to explore," said Vandekerkhove, who isn't involved in the study.
"Being a Canadian-led initiative is really great for Canadian researchers."
She also flagged the trial's limits.
"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."
A patient's story
Paul Lonergan believes the research has already changed his life.
The 68-year-old Toronto man was diagnosed with throat cancer about three years ago. An avid hockey player, Lonergan said his family doctor initially thought he had a virus, until he started "coughing up blood on the ice."
He was referred to an ear, nose and throat specialist, then to Princess Margaret Cancer Centre, where he went through radiation and chemotherapy.
Lonergan was also enrolled in a separate clinical trial called MERIDIAN, which looked for residual cancer in the blood of patients 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.
He spent several more months on 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 still has trouble swallowing and relies on smoothies rather than solid food, but he's back on the ice as he continues to recover.
"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."
Funding and next steps
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.
