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 the "liquid biopsy" can detect genetic changes in the tumours of patients and indicate if they are less likely to respond to a new targeted drug.
These genetic changes can also predict whether 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 resistant to treatment is the greatest challenge we face 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 to respond with adaptable new treatment plans."
Fragments of cancer DNA in the blood samples of 310 women with oestrogen receptor positive breast cancer - the most common form of the disease - were analysed during the study.
The patients had advanced breast cancer and were taking part in a trial of targeted drug palbociclib and hormone therapy fulvestrant.
The research team found 42 per cent of the women had one or more of three changes in the tumour DNA in their bloodstream that put them at risk of early relapse.
Women whose circulating tumour DNA contained changes in the cancer gene p53 saw their cancer return after an average of 3.7 months, compared to 12.7 months among women without the gene changes. 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 study found that a new genetic test could detect right at the start of treatment those women whose cancers were most likely to develop resistance quickly to palbociclib.
"We could then adjust their treatment plan accordingly - trialling additional treatments from the outset to try and prevent resistance, or planning for a switch to another treatment as soon as resistance develops.
"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.
