Blood Test May Detect Cancer Recurrence Before It Appears on Scans

Researchers in Toronto are launching a major study that could change how cancer patients are monitored after treatment, by testing whether a simple blood test can detect early signs of cancer returning before they appear on traditional medical scans.

According to The Canadian Press, researchers at Princess Margaret Cancer Centre are working on a large trial to determine whether a blood test can identify tiny amounts of cancer that may remain in the body after patients complete treatment. Lead investigator Dr. Lillian Siu said smaller studies have shown that tumour DNA can sometimes appear in the blood in amounts too small to be detected by CT scans.

The trial, called SHERLOCK, is enrolling about 7,000 patients who have completed radiation, chemotherapy or other cancer treatments. Researchers will test their blood for microscopic traces of tumour DNA, a sign known as molecular residual disease. The patients are expected to be followed for at least five years to determine whether the test can reliably predict longer-term outcomes.

The test is often described as a liquid biopsy. Instead of removing tissue from the body, it looks for fragments of cancer DNA circulating in the bloodstream. The Ontario Institute for Cancer Research says these fragments, known as circulating tumour DNA or ctDNA, can sometimes be present in blood before a tumour becomes visible on a scan, making them a potentially important marker for earlier cancer monitoring.

For patients, the promise is significant but still under investigation. If the test is positive, doctors may be able to consider additional experimental treatments, including new immunotherapies, before the cancer becomes visible again. If the test is negative, it may one day help some patients avoid unnecessary chemotherapy or radiation and reduce the side effects of treatment they may not need.

Researchers are careful not to overstate the findings. Dr. Siu emphasized that blood tests used to predict cancer recurrence are promising, but they are not yet part of standard medical care. Larger clinical trials such as SHERLOCK are needed to prove how well the technology works across different types of cancer and whether it can truly improve patient outcomes.

The study also aims to answer an important scientific question: does the accuracy of the blood test vary from one cancer type to another? That matters because much of the previous research in liquid biopsy has focused on specific cancers, while SHERLOCK is designed to collect broader evidence across multiple tumour types.

ClinicalTrials.gov describes SHERLOCK as a platform study using ctDNA to analyze molecular residual disease in patients treated with curative intent for cancer. The trial will also collect clinical information about patients, their disease, treatment and outcomes to help researchers better understand how these blood-based signals should be interpreted.

For many cancer survivors, the period after treatment is filled with uncertainty. A clear scan can bring relief, but the fear of recurrence often returns before the next appointment. If this research proves successful, a blood test could become a powerful tool to help doctors personalize follow-up care, intervene earlier when needed, and reassure patients when there is no molecular sign of disease.

For now, the message remains cautious but hopeful: the test is not a replacement for medical scans or standard follow-up care today, but it may represent an important step toward detecting cancer recurrence earlier and making post-treatment care more precise.