A new blood test could help predict whether women with breast cancer will respond to treatment before it begins.
Scientists at London's Institute of Cancer Research said "liquid biopsy" can detect genetic changes in patients and indicated if they are less likely to respond to new targeted drugs.
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These genetic changes can also predict whether or not a patient's disease is likely to return.
Professor Paul Workman, chief executive of the Institute of Cancer Research, said: "Cancer's ability to evolve to become the greatest challenge in improving patients' survival and quality of life.
"So-called liquid biopsy tests like this one are a key part of our toolkit in staying on top of cancers' adaptability and evolution, and picking up the earliest signs of drug resistance.
"Detecting the potential of cancers to evolve resistance could allow us to predict cancer's next move and respond with adaptable new treatment plans."
Fragments of cancer DNA samples of women during the study were fragmented during the study.
The patients had advanced breast cancer and were taking part in a trial of targeted drug palbocyclib and hormone therapy fulvestrant.
The research team found 42 for the center of women with one or more of three changes in their bloodstream that put them at risk of early relapse.
Women whose circulating tumors DNA changes in the p53 gene An increase in other genes was also found to predict when the cancer might return.
Professor Nicholas Turner, from the Institute of Cancer Research and Royal Marsden, said the researcher said that he was more likely to develop resistance quickly to palbociclib.
"We could then adjust their treatment plan accordingly – trialling additional treatments to preventive resistance, or planning for a switch to another treatment as soon as resistance development.
"We now need to assess in a clinical trial whether helping direct women's care with this new test can offer improved survival and quality of life."
The research was presented at the American Society of Clinical Oncology's annual meeting in Chicago.