Why This Tiny DNA Clue Could Change Breast Cancer Care

A routine blood draw may soon answer one of the hardest questions in breast cancer care: which treatment will actually work for a specific patient.

Scientists have developed a DNA-based blood test that can predict how well someone with breast cancer will respond to treatment, sometimes before the first drug is even given, and again after just four weeks.

The findings suggest doctors could avoid ineffective therapies early, switch strategies faster, and give patients a better chance of keeping the disease under control.

Breast cancer affects more than 2 million people worldwide every year, making it the most commonly diagnosed cancer globally, according to the WHO. While treatment options have expanded in recent decades, matching the right therapy to the right patient remains a major challenge.

Many people endure months of side effects from drugs that ultimately do little to slow their cancer.

This new test aims to change that.

Cancer Leaves Clues in the Bloodstream

The approach relies on a liquid biopsy, a blood test that looks for tiny fragments of circulating tumour DNA, or ctDNA. These DNA fragments are released into the bloodstream by cancer cells and can be detected with highly sensitive sequencing techniques.

Researchers analyzed blood samples from 167 patients with advanced breast cancer, measuring ctDNA levels before treatment began and again after one treatment cycle, roughly four weeks later.

What they found was striking: lower ctDNA levels consistently predicted better outcomes.

Patients with low ctDNA at the start of treatment were more likely to respond to therapy. In many cases, their cancer stayed under control for significantly longer than those with higher ctDNA levels.

The same pattern appeared when ctDNA was measured after four weeks, often with even clearer separation between those who benefited and those who did not.

A Clear Signal in an Aggressive Cancer Type

The strongest results were seen in people with triple-negative breast cancer, an aggressive form that accounts for about 10–15% of cases worldwide and lacks obvious drug targets.

Among these patients, those with low ctDNA levels before treatment saw their cancer controlled for about 10 months, compared with just over 4 months for those with higher levels.

Tumour shrinkage occurred in 40% of patients with low ctDNA, but in fewer than 10% of those with higher readings.

After four weeks of treatment, the difference became even sharper. Patients whose ctDNA was no longer detectable had their cancer held at bay for roughly a year, while those with persistent ctDNA saw progression in just a few months.

Why This Could Change How Cancer Is Treated

The promise of this test is not just prediction; it is speed.

Instead of waiting months to see whether a treatment is working, doctors could know within weeks, or even before therapy begins.

That opens the door to switching drugs earlier, combining treatments, or enrolling patients in clinical trials before the disease has time to advance.

For patients, this could mean fewer unnecessary side effects, less time lost on ineffective therapies, and more personalized care from the outset.

While the study focused on advanced breast cancer, researchers believe the same approach could also work in earlier-stage disease, where treatment decisions can have long-lasting consequences.

A Glimpse of Personalized Cancer Care

The test is still being evaluated in clinical trials to confirm whether adjusting treatment based on early ctDNA results improves survival and quality of life.

But the signal is strong: a simple blood test may soon guide some of the most critical decisions in cancer care.

If validated at scale, liquid biopsies like this could mark a shift away from trial-and-error treatment, toward faster, smarter, and more precise cancer therapy tailored to each patient’s biology.

Long-term population data already show progress, with breast cancer mortality down by 44% since peak years, a trend tied to earlier detection and smarter treatment choices.