Blood test detects breast cancer more than a year before scans

A quiet trace of cancer can linger in the body long after treatment ends. It may stay hidden for months or even years before doctors can see it on a scan or before symptoms begin to appear. Now, researchers at Lund University say they have developed a blood test capable of spotting those warning signs far earlier than current methods.

The study followed breast cancer patients over several years and tracked tiny fragments of tumor DNA circulating in their blood. Researchers found that the test could identify signs of recurrence a median of 13.8 months before the cancer became clinically visible. In some patients, the signal appeared nearly four years earlier.

“This study shows that our blood-based method, Pathlight, which reliably measures small fragments of tumor DNA, can provide early information about how breast cancer responds to chemotherapy before surgery while also indicating whether the disease has come back after surgery,” said Lao Saal, researcher at Lund University and senior author of the study.

The findings could reshape how doctors monitor breast cancer after treatment. Instead of waiting for tumors to grow large enough to appear on imaging scans, physicians may eventually be able to track hidden cancer activity through a simple blood draw.

Doctors already use several tools to measure breast cancer response. One important marker is called pathological complete response, or pCR. This means no invasive cancer remains after chemotherapy and surgery.

Patients who achieve pCR often have better outcomes. But the measure does not work perfectly for everyone. Some patients who fail to achieve pCR still remain cancer-free for years. Others experience recurrence despite aggressive treatment.

Researchers have increasingly turned to circulating tumor DNA, known as ctDNA, for answers. Tumors release tiny pieces of DNA into the bloodstream as cells break apart. Scientists can analyze those fragments to look for signs of residual disease.

The Lund University team wanted to test whether ctDNA could predict recurrence more accurately and earlier than current monitoring methods.

“The method is based on the idea that every tumor has a unique genetic fingerprint,” said Saal. “By measuring these alterations in the patient’s blood, we can detect extremely small amounts of residual tumor DNA with high precision, even when a recurrence is not yet visible with today’s imaging methods or has begun causing symptoms.”

To build the test, researchers first analyzed tumor tissue from each patient. They searched for structural variants, which are large DNA rearrangements unique to each cancer.

These genetic alterations act like fingerprints. Once identified, scientists created personalized blood tests capable of tracking those same DNA signatures over time.

The researchers used digital PCR, a highly sensitive laboratory technique that counts DNA molecules with remarkable precision. The test could track up to 16 structural variants for each patient.

The sensitivity was striking. In some cases, tumor DNA levels measured as low as 0.000136 percent, roughly equal to 1.36 parts per million. Analytical testing detected structural variants at levels as low as 0.00001 percent.

The method also achieved 100 percent analytical specificity in thousands of control measurements, meaning false positives remained extremely rare.

Unlike more complex sequencing methods, the system required less computational analysis and could process blood samples more rapidly and cost-effectively.

The study included 136 patients treated with chemotherapy and surgery for different forms of breast cancer. Blood samples were collected at diagnosis, during treatment, shortly after surgery and regularly during follow-up for as long as six years.

The median patient age was 52, although participants ranged from 24 to 78 years old. About 44.9 percent had hormone receptor-positive and HER2-negative disease. Another 29.4 percent had triple-negative breast cancer, while 24.3 percent had HER2-positive disease.

Researchers collected nearly 1,500 blood samples during the study.

Before treatment began, tumor DNA was detectable in nearly 90 percent of patients. Detection rates varied by cancer subtype. All patients with triple-negative breast cancer had detectable ctDNA at diagnosis. Detection reached 90.9 percent in HER2-positive disease and 82 percent in hormone receptor-positive and HER2-negative disease.

For most patients, ctDNA levels dropped during chemotherapy. But not everyone responded the same way.

In about 21 percent of patients, tumor DNA remained detectable after chemotherapy before surgery. Those patients faced a much higher risk of recurrence and death.

Researchers found that patients with persistent ctDNA had more than triple the risk of recurrence compared with patients whose ctDNA disappeared. Their risk of death also rose sharply.

The dynamic pattern of ctDNA changes proved even more powerful than single measurements. Researchers defined a ctDNA response as either complete clearance or at least a 50 percent reduction from baseline levels.

Patients who failed to show this response had dramatically worse outcomes. More than half later experienced recurrence. In contrast, only about 10 percent of responders relapsed.

The association appeared especially strong in triple-negative breast cancer, one of the most aggressive forms of the disease.

Importantly, ctDNA patterns outperformed pathological complete response in predicting long-term outcomes.

After surgery, researchers continued monitoring patients for signs of molecular residual disease. If ctDNA appeared during follow-up, recurrence risk increased dramatically.

Among patients who later developed distant metastatic disease, ctDNA predicted recurrence in 86.7 percent of cases. When researchers examined only pathologically confirmed metastases, sensitivity rose to 92.9 percent.

In some patients, the blood test detected returning cancer nearly four years before doctors observed clinical relapse.

One example involved a 75-year-old woman with HER2-positive and hormone receptor-positive disease. Her ctDNA disappeared during treatment and after surgery. Eighteen months later, the tumor DNA signal returned even though scans still appeared normal. Clinical relapse did not occur until almost four years later.

Another patient with triple-negative breast cancer showed rising ctDNA during chemotherapy. The levels stayed positive after surgery. Thirteen months later, clinical recurrence appeared.

Researchers also found that higher ctDNA quantities predicted poorer survival. Every doubling of ctDNA levels increased recurrence and death risk.

The researchers believe ctDNA monitoring could eventually help doctors personalize treatment decisions more effectively.

Patients with persistent ctDNA may benefit from more aggressive follow-up or additional therapies. Meanwhile, patients with consistently negative ctDNA might avoid unnecessary treatments and their side effects.

“These findings could change how breast cancer patients are monitored and treated in the future,” said Niklas Loman, senior oncologist at Skåne University Hospital and one of the study researchers.

“With further research, the technology may provide opportunities to improve treatment for patients at high risk of recurrence. The method may also help us optimize treatment for patients at the lowest risk of recurrence who may be able to avoid unnecessarily intensive therapy, potentially leading to fewer side effects.”

Still, the researchers caution that the test is not yet ready for routine clinical use. Some patients with positive ctDNA never developed visible recurrence during follow-up. Scientists say immune activity or ongoing treatment effects may explain why some microscopic disease never progresses.

Future clinical trials will determine whether changing treatment based on ctDNA results can improve survival outcomes.

These resources explore molecular residual disease, ctDNA surveillance and the challenges of turning earlier detection into better cancer outcomes.

Circulating Tumor DNA in Early Breast Cancer: A Review: Reviews emerging evidence for using ctDNA to detect residual disease, monitor treatment and identify molecular recurrence before conventional methods. (JAMA Oncology, 2026)

Ultrasensitive Detection and Monitoring of Circulating Tumor DNA Using Structural Variants in Early-Stage Breast Cancer: Evaluates structural-variant tracking as an ultrasensitive tumor-informed approach for monitoring early breast cancer and detecting recurrence. (Clinical Cancer Research, 2025)

Longitudinal monitoring of circulating tumor DNA to detect relapse early and predict outcome in early breast cancer: Shows how repeated personalized ctDNA measurements can identify molecular residual disease before clinical relapse in high-risk breast cancer. (npj Precision Oncology, 2024)

A standing platform for cancer drug development using ctDNA-based evidence of recurrence: Examines how molecular evidence of recurrence could be used to test treatments before radiographic disease becomes visible. (Nature Reviews Cancer, 2024)

Bridging the gap: ctDNA, genomics, and equity in breast cancer care: Reviews the opportunities and practical barriers surrounding ctDNA for profiling, residual disease detection and treatment monitoring in breast cancer. (npj Breast Cancer, 2025)

Research findings are available online in the journal EMBO Molecular Medicine.

The original story "Blood test detects breast cancer more than a year before scans" is published in The Brighter Side of News.

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