Blood test could detect cancer recurrence before CT scans, Toronto researchers find

At Princess Margaret Cancer Centre in Toronto, researchers are testing whether a simple blood draw could answer one of the most haunting questions a cancer survivor faces: Is it really gone?

Dr. Lillian Siu and her team are running a trial called SHERLOCK, enrolling 7,000 patients who have finished their radiation, chemotherapy, or other cancer treatments. The researchers are looking for something invisible to standard imaging—fragments of tumour DNA circulating in the bloodstream in quantities so small that CT scans cannot detect them. This approach, known as a liquid biopsy, has shown promise in smaller studies around the world, but Siu's team is conducting the large-scale validation needed to prove it works across different cancer types and patient populations.

The logic is straightforward but powerful. If a blood test finds these microscopic remnants of cancer—a condition researchers call molecular residual disease—patients could receive experimental immunotherapies designed to prevent recurrence before any tumour becomes visible on imaging. If the test comes back negative, it could offer something equally valuable: confirmation that the cancer is truly gone, allowing doctors to spare patients from additional rounds of chemotherapy or radiation that carry their own serious side effects. Siu emphasizes that the trial will 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)," she said. "Just stopping at one year is not going to be sufficient."

The psychological weight of cancer recurrence anxiety is real and persistent. Siu has watched it play out in her clinic countless times. Patients arrive for follow-up appointments carrying visible fear. When their scans come back clear, there is relief—but temporary. The anxiety returns at the next appointment, and the cycle repeats. "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," Siu said. Many wonder when they will finally be free from the constant dread that their cancer will return.

The SHERLOCK trial has already attracted attention from researchers studying liquid biopsies in other countries. Gillian Vandekerkhove, an assistant professor at the University of British Columbia who studies bladder cancer and liquid biopsies, welcomed the trial's broad scope across multiple cancer types. "They're going to provide a wealth of information and biobank samples that researchers can continue to explore," she said. But Vandekerkhove also sounded a note of caution. This is an observational study designed to help researchers understand the technology and identify its best applications, not a definitive proof that it should move into routine clinical practice. Additional trials will be needed before that happens.

Paul Lonergan, a 68-year-old Toronto man, has already experienced the potential benefit firsthand. Three years ago, he was diagnosed with throat cancer after initially being told by his family physician that he had a virus. The symptoms escalated—he began coughing up blood while playing hockey, his lifelong passion. After treatment at Princess Margaret with radiation and chemotherapy, Lonergan was enrolled in a different trial called MERIDIAN, which tested for residual cancer DNA in head and neck cancer patients. The results changed his trajectory. "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 recalled. He received several months of immunotherapy treatment. "Sure as heck it worked," he said. After three consecutive six-month checkups, his doctors cleared him. He still struggles with swallowing and relies on smoothies rather than solid food, but he has returned to the ice. "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 trial is funded by a $50-million donation from the Peter Gilgan Foundation and represents a significant institutional commitment to validating this technology. But Siu is careful to manage expectations. Blood tests for predicting cancer recurrence are not yet standard of care. The research is promising, but the evidence must be built methodically. The trial will take years to complete, and the field will need to see the results before this becomes a routine tool in cancer follow-up care.