You Finished Cancer Treatment. Your Scans Are Clear. So Why Does the Fear Never Fully Leave?

You finish treatment. Your scans come back clear. Your doctor says you are doing well. And yet, for a lot of cancer survivors, one question stays in the back of the mind for years afterward: what if it comes back? A new generation of blood tests focused on liquid biopsy cancer recurrence detection is trying to answer that question earlier than scans ever could, and some of the most important research on it is happening right here in Canada.

What Is a Liquid Biopsy, Actually

Most people have a rough idea of what a biopsy is: doctors take a small piece of tissue and check it for cancer cells. A liquid biopsy flips that idea. Instead of removing tissue, it uses a simple blood draw to search for fragments of tumour DNA that cancer cells shed into the bloodstream as they grow or die.

Here is the part that matters for survivors specifically. Even after treatment has wiped out every visible sign of a tumour on a scan, some of those DNA fragments can still be floating in the blood, and their presence signals the cancer has not actually been eliminated. Catching that signal early, before anything shows up on imaging, could let doctors act sooner and with more treatment options on the table. Doctors call this circulating tumour DNA, or ctDNA for short: it is simply the genetic material that tumours leave behind in the blood, and it is what these tests are built to detect.

The phrase "liquid biopsy" was first used back in 2010, and the underlying technology has improved steadily since. What once required a large blood draw to pick up a trace signal can now be done with a standard sample, thanks to what researchers describe as ultra-sensitive detection methods.

Why This Actually Matters if You Have Had Cancer

Standard follow-up care after treatment usually means periodic scans and clinic visits, and those tools have a real limitation: they can only spot cancer once it has grown large enough to be visible. By that point, treatment options can already be narrower.

This is where liquid biopsies could change the picture. Dr. Lillian Siu, a senior scientist at Toronto's Princess Margaret Cancer Centre and a scientific lead at the Peter Gilgan Centre for Early Cancer Detection Research, describes it as taking detection one step earlier: catching cancer while it is still only detectable in the blood, before it shows up on a routine exam or CT scan. "The hope, obviously, is when you detect it so early, we can do something about it earlier, and hopefully offer these patients a better chance at receiving curative treatment," Dr. Siu told CBC News.

Putting this into everyday clinical care, rather than only using it inside research studies, is still very much a work in progress. That distinction, between a promising research tool and something your own oncologist can order tomorrow, is one worth holding onto as you read the rest of this.

The Canadian Trial Making Noise Right Now

A Toronto-based research team has launched one of the most ambitious liquid biopsy studies in Canada to date, called the SHERLOCK trial, short for Study of High-Precision Evaluation of Molecular Residual Disease Through a Platform for Cancer Tracking and Interception. Molecular residual disease is the technical term for exactly what it sounds like: tiny amounts of cancer left behind after treatment that are too small for a scan to catch but that a blood test might still detect. Led by Dr. Siu's team at Princess Margaret Cancer Centre alongside the Peter Gilgan Centre for Early Cancer Detection Research, SHERLOCK plans to offer liquid biopsies to more than 7,000 people who have already completed curative treatment for a range of cancer types, running until 2031.

The goal is to build hard clinical evidence on whether these tests can actually guide what happens next for a patient: whether someone needs additional treatment, or whether someone else can safely skip it. That second half matters just as much as the first. Overtreatment is a genuine problem in cancer care, and liquid biopsies could spare some survivors from aggressive follow-up treatment when a test shows no trace of disease. When a test does detect a warning sign, participants are connected to other clinical trials offering access to newer therapies, including experimental immunotherapy.

Gillian Vandekerkhove, a University of British Columbia researcher who studies liquid biopsies and was not involved in SHERLOCK, called the Canadian-led scale of the project valuable for researchers here, while also noting an important caveat: SHERLOCK is an observational study, meaning it will teach researchers a great deal about how the technology performs, but further trials will still be needed before liquid biopsies become a standard part of clinical care.

A Real Patient's Experience With This Technology

Paul Lonergan, a Toronto resident, was 68 when he was diagnosed in 2023 with a large tumour at the base of his tongue. Seven weeks of combined radiation and chemotherapy, intensive enough to land him in intensive care, left his scans clear.

But was it really gone? Through a clinical trial, Lonergan received a liquid biopsy after treatment, and the result was sobering: tumour DNA was still circulating in his blood, suggesting the cancer could return even though his scans looked clean. That finding opened the door to experimental immunotherapy, and his liquid biopsies since have shown no trace of disease. His case is a precise, human version of what researchers hope this technology can do at scale: catch the signal that scans miss, then act on it while there is still time.

What This Technology Cannot Do Yet

It is worth being honest about the current limits, because this field is still maturing. Liquid biopsies can produce false positives, where the test flags tumour DNA but no active disease actually exists, and false negatives, where the blood looks clear even though cancer is returning. Neither happens rarely enough to ignore.

Dr. Christopher Booth, a medical oncologist and health services researcher at Queen's University in Kingston, raises a different kind of consideration: the emotional weight of knowing. If your scans are clear and you feel healthy, do you actually want a blood test that might tell you something is still wrong? "Some people would want to know because they can plan their life accordingly," Booth told CBC News. "Other patients might not want to know. And then you add on top of that the possibility of a false negative or a false positive test, and there can be significant emotional burden." That is not a small question, and there is no universally correct answer to it.

These tests are not one-time events either. Useful monitoring means repeating them at intervals, and researchers are still working out the right timing for different cancer types. Cost is a real barrier too: private liquid biopsy testing currently runs around $5,000 per test in some cases, out of reach for most Canadians. Researchers hope large trials like SHERLOCK will build the evidence base needed to bring these tests into the publicly funded system.

Where Things Stand in Canada Right Now

A small number of Canadian hospitals are already using liquid biopsy technology clinically. New Brunswick announced access to liquid biopsies for lung cancer monitoring in 2024, and at least one Ontario hospital uses the technology diagnostically. For most cancer survivors across the country, though, access still depends on qualifying for a clinical trial or being able to pay privately.

Dr. Ramy Saleh, a cancer researcher and oncologist at the McGill University Health Centre in Montreal, put the underlying challenge plainly: figuring out how the public healthcare system can pay for or cover these costs. Part of what SHERLOCK is designed to demonstrate is whether widespread liquid biopsy use could ultimately save the healthcare system money, by cutting unnecessary treatment and catching recurrences earlier, when they are cheaper and simpler to treat. Building the evidence base for trials like this is also part of a broader push in Canada to strengthen research infrastructure, including a federal investment in making health data more accessible to researchers while keeping patient identities protected.

If you or someone you love has been through cancer treatment, liquid biopsies are not yet standard follow-up care for most Canadians. But that is shifting, and Canadian research sits close to the centre of that shift.

What is a liquid biopsy?

A liquid biopsy is a blood test that searches for tiny fragments of tumour DNA that cancer cells shed into the bloodstream. Unlike a traditional biopsy, it does not require removing tissue, and it can sometimes detect cancer activity before anything shows up on a scan.

How is a liquid biopsy different from a regular cancer scan?

Scans like CT or MRI can only detect a tumour once it has grown large enough to be visible. A liquid biopsy looks for circulating tumour DNA in the blood, which can sometimes be present before a tumour is large enough to appear on imaging, offering an earlier warning sign in some cases.

Is liquid biopsy testing available to cancer patients in Canada right now?

In a limited way. A few hospitals, including sites in New Brunswick and Ontario, use it clinically for specific cases like lung cancer monitoring. For most people, access currently depends on qualifying for a clinical trial such as SHERLOCK, or paying privately, which can cost around $5,000 per test.

What is the SHERLOCK trial?

SHERLOCK is a Canadian clinical trial based at Toronto's Princess Margaret Cancer Centre, led by Dr. Lillian Siu, offering liquid biopsies to more than 7,000 people who have completed cancer treatment. It runs until 2031 and aims to determine whether the tests can reliably guide follow-up treatment decisions.

Can a liquid biopsy be wrong?

Yes. These tests can produce false positives, suggesting cancer DNA is present when no active disease exists, and false negatives, showing clear blood when cancer is actually returning. Because of this, researchers are still studying how liquid biopsies should be used and interpreted in routine follow-up care.

Should I ask my oncologist about a liquid biopsy?

If you are curious whether you might qualify for a clinical trial involving liquid biopsy testing, raising it with your oncologist or specialist is a reasonable starting point. They can tell you whether it is relevant to your specific cancer type and current stage of follow-up care.

If you or someone you love has finished cancer treatment and wants to explore whether a clinical trial involving this kind of testing might be relevant, a conversation with your oncologist or specialist is the best starting point. This article shares general health information and current research, not a personal medical recommendation; any decisions about your follow-up care should be made with your care team. If you need help finding an oncologist or specialist in your area, Medimap can help you book at medimap.ca, and you can find more Canadian health research explainers on the Health Hub.