A groundbreaking Canadian research initiative is exploring whether a simple blood test can accurately determine if cancer is likely to return after treatment, offering hope to thousands of cancer patients living with the fear of recurrence.
Researchers at Princess Margaret Cancer Centre in Toronto have launched one of the world’s largest clinical trials, known as SHERLOCK, to investigate whether tiny traces of tumour DNA circulating in the bloodstream can reveal the presence of microscopic cancer cells long before they are detectable through conventional imaging such as CT scans.
The study, led by Dr. Lillian Siu, aims to enroll approximately 7,000 patients who have completed treatment for various types of cancer, including radiation, chemotherapy and surgery. Participants will undergo regular blood testing over a period of at least five years to determine whether the presence of circulating tumour DNA accurately predicts the return of cancer.
Previous smaller studies conducted worldwide have shown promising results, suggesting that fragments of cancer DNA can remain in the bloodstream even when scans appear clear. Researchers believe these microscopic traces, known as molecular residual disease, could provide an earlier warning that cancer is returning.
If a patient’s blood test is positive, doctors may be able to offer additional experimental therapies, including newer immunotherapy treatments, in an effort to eliminate the remaining cancer cells before the disease progresses. Conversely, patients with negative blood tests may be able to avoid unnecessary chemotherapy or radiation, reducing treatment-related side effects while gaining greater confidence that their cancer has been successfully eliminated.
Dr. Siu emphasized that although the technology shows considerable promise, liquid biopsy testing is not yet considered standard medical practice. She noted that large clinical trials such as SHERLOCK are essential before the test can become part of routine cancer care.
Researchers also hope the study will determine whether the blood test performs equally well across different types of cancer, providing valuable information that could guide future treatment strategies.
One of the greatest motivations behind the research is addressing the emotional burden carried by cancer survivors. According to Dr. Siu, many patients experience ongoing anxiety during every follow-up visit, fearing that their cancer may return despite successful treatment. The ability to accurately identify patients who are truly cancer-free could significantly reduce that uncertainty.
The study has also attracted praise from researchers across Canada. Dr. Gillian Vandekerkhove, an assistant professor at the University of British Columbia who specializes in bladder cancer and liquid biopsies, said the nationwide, multi-cancer approach will generate valuable scientific data and create an important biobank for future cancer research. However, she cautioned that SHERLOCK is an observational study and additional clinical trials will still be required before the technology can be widely adopted in hospitals.
For Toronto resident Paul Lonergan, the promise of liquid biopsy research has already become a reality. Diagnosed with throat cancer about three years ago after initially being treated for what was believed to be a viral infection, Lonergan underwent radiation and chemotherapy at Princess Margaret Cancer Centre.
Following treatment, he participated in another clinical trial called MERIDIAN, which monitored blood samples for residual cancer DNA in patients with head and neck cancers. Although his tumour had disappeared, the blood test detected lingering cancer fragments that would not have been visible through imaging.
Doctors subsequently enrolled him in an immunotherapy trial, where he received additional treatment over several months. Today, after multiple follow-up examinations, Lonergan remains cancer-free.
Although he still experiences difficulty swallowing and relies mainly on smoothies instead of solid food, he has returned to playing recreational hockey and says the experience has given him a renewed appreciation for life.
The SHERLOCK clinical trial has been made possible through a $50 million donation from the Peter Gilgan Foundation and is expected to provide important evidence that could transform how doctors monitor cancer patients after treatment. If successful, the research could pave the way for earlier intervention, more personalized care and greater peace of mind for cancer survivors across Canada.