A simple blood test could soon help doctors track breast cancer more closely and reduce reliance on repeated tumour biopsies, following a breakthrough by researchers in Sweden who have developed a new way to detect tiny traces of cancer DNA in the bloodstream that existing methods often miss.

Scientists from Chalmers University of Technology and the University of Gothenburg have unveiled a statistical analysis method called "BayesCNA" that can identify tumour-related DNA, even when it makes up only about 5 per cent of the total DNA found in a blood sample. Current approaches generally require cancer DNA levels of 15–20 per cent to produce detailed results, limiting their usefulness once treatment begins to work and tumour DNA levels start falling.

The advance comes at a time when blood-based cancer testing, often known as liquid biopsy, is attracting growing attention worldwide. Researchers and clinicians hope such tests could eventually offer a less invasive alternative to repeated tissue sampling, particularly for women undergoing breast cancer treatment.

“We wanted to develop a method that works particularly well in difficult cases where there is very little cancer DNA in the blood and a lot of what we consider noise—that is, mainly healthy DNA. Our results indicate that the new method performs better with samples involving low levels of cancer DNA, where the proportion is around 5 per cent. So, it works exactly as we had hoped,” said Lotta Eriksson, a doctoral student involved in the research.

Cancer cells continuously shed fragments of their genetic material into the bloodstream. These fragments, known as circulating tumour DNA, can reveal how a cancer is evolving. The challenge is that the signal is often fragile. Finding it can be like spotting a whisper in a crowded room.

The Swedish team used low-pass whole-genome sequencing, a relatively affordable technique that provides a broad overview of DNA rather than a detailed readout. Eszter Lakatos, assistant professor in mathematical sciences, compared it to skimming through a book rather than reading every page. To compensate for the limited information, the researchers developed a sophisticated statistical algorithm that amplifies faint cancer signals hidden within large amounts of normal DNA.

“When the treatment is effective, the amount of cancer DNA in the blood drops significantly. This makes it more difficult both to detect the cancer and to monitor how it changes. It is important to be able to analyse samples containing low levels of cancer DNA to gain a clearer picture of how a patient responds to treatment,” Lakatos said.

The findings align with a growing body of evidence from organisations such as the US National Cancer Institute and studies published in Nature Medicine, The Lancet Oncology, and Cancer Discovery that have demonstrated the potential of liquid biopsies to guide precision oncology. Precision oncology refers to tailoring treatment according to the unique genetic features of a patient's tumour rather than relying solely on standard treatment protocols.

For patients, the implications could be significant. A woman may undergo surgery only once or twice during treatment, but blood samples can be collected every few weeks. Such regular monitoring could help doctors detect early signs of treatment resistance and adjust therapies more quickly.

“A patient may undergo surgery once or twice, whereas blood tests may be taken at intervals of just a few weeks during treatment. If we can obtain information about tumour changes from the samples, we can monitor developments much more closely and see what happens between treatment sessions. This can help doctors make more informed decisions, such as tailoring treatment to the tumour's composition,” Lakatos added.

The researchers are now working to identify additional tumour characteristics hidden within the blood samples, and they hope to validate the technology in future clinical trials. While the method is not yet ready for routine hospital use, experts say it represents another important step towards more personalised, less invasive cancer care.