Blood test shows promise in detecting cancer recurrence before symptoms appear

At Princess Margaret Cancer Centre in Toronto, researchers are testing a hypothesis that has quietly animated cancer medicine for the past decade: that the body's blood can reveal what imaging cannot. The test is called a liquid biopsy, and it hunts for fragments of tumour DNA so small that CT scans miss them entirely. If it works at scale, it could transform how doctors manage the fear that haunts every cancer survivor—the fear that the disease is still there, waiting.

Dr. Lillian Siu, the lead investigator, is enrolling 7,000 patients who have completed radiation, chemotherapy, or other cancer treatments into a trial called SHERLOCK. The researchers will draw blood from each patient and search for microscopic quantities of cancer DNA. Smaller studies around the world have already shown that this is possible. What remains unknown is whether it matters—whether detecting these molecular remnants actually predicts which patients will relapse, and whether catching them early enough allows doctors to intervene before the cancer returns.

The logic is straightforward. If the blood test comes back positive, patients could receive experimental immunotherapies designed to eliminate the remaining cancer cells before they multiply. If the test is negative, it offers something equally valuable: permission to stop treatment. Many cancer survivors endure months or years of additional chemotherapy or radiation as a precaution, accepting the cumulative damage to their bodies in exchange for the illusion of safety. A negative liquid biopsy could spare them that burden.

Siu emphasizes that this is not yet standard care. The data from earlier studies is promising—patients with detectable cancer DNA in their blood have a very high likelihood of relapse—but promising is not the same as proven. SHERLOCK will follow its 7,000 patients for at least five years. Siu is clear about why: one year of follow-up is not enough. You need to know whether the test actually predicts long-term outcomes, whether it changes the trajectory of disease, whether it saves lives.

The trial will also examine whether the blood test performs equally well across different cancer types, a question that has not been thoroughly answered. Gillian Vandekerkhove, a bladder cancer researcher at the University of British Columbia, welcomed this broad approach. She also tempered expectations. SHERLOCK is an observational study, she noted—it will help researchers understand the technology better, but additional trials will be needed before blood tests for cancer recurrence become routine clinical practice.

Paul Lonergan, a 68-year-old from Toronto, did not wait for SHERLOCK to enroll him. Three years ago, he was diagnosed with throat cancer after coughing up blood on the ice during a hockey game. After radiation and chemotherapy, he joined a different trial called MERIDIAN, which tested for residual cancer in head and neck patients. His blood test came back positive for cancer fragments. His doctor offered him an experimental immunotherapy drug. "Sure as heck it worked," Lonergan said. Three six-month checkups later, he remains cancer-free. He still struggles to swallow and drinks smoothies instead of solid food, but he is back on the ice, playing hockey for the simple pleasure of it.

What Siu sees in her clinic every day is the psychological weight that Lonergan's story illustrates. Cancer patients come back for follow-up appointments carrying fear in their eyes. They breathe relief when their CT scans are clear, then the anxiety returns at the next scan. When am I actually free from this? they wonder. A blood test that could answer that question—that could either confirm the cancer is truly gone or catch it before symptoms appear—addresses one of the deepest anxieties in cancer care. The SHERLOCK trial, funded by a $50 million donation from the Peter Gilgan Foundation, is designed to determine whether that answer is reliable enough to change how medicine is practiced.