Breast cancer tumours can evolve to "make their own fuel", rendering the drugs used to treat them useless, a study has found.
Scientists had previously thought treatments stopped working in some patients as cancers developed resistance is some way, but did not know how.
New research suggests some tumours begin to make their own oestrogen "fuel supply" as drugs target the hormone.
An international team, led by Imperial College London and the European Institute of Oncology in Milan, hope its findings will increase treatment options for patients whose cancer has returned or spread.
"Once a cancer spreads, the disease is incurable. However, let's not give up this fight too soon - if we take a second biopsy we can find out which treatments would work," research co-author Dr Luca Magnani said.
Breast cancer is the most common form of the disease in the UK with around 55,000 cases each year.
Around 70 per cent of sufferers are so-called ER positive, meaning the cancer cells are fuelled by the hormone oestrogen, and these patients are offered one of two treatments after surgery to stop the cancer coming back.
The drug tamoxifen and aromatase inhibitors act in different ways to cut off the oestrogen supply, but both stop working in around one-third of patients.
The latest findings, which are published in the journal Nature Genetics, show that in one-quarter of patients taking aromatase inhibitors, the tumours had increased production of aromatase in the cancer cells.
Scientists believe the tumours do this by increasing the number of aromatase genes, allowing the cancer cells to effectively make their own oestrogen.
"For the first time we have seen how breast cancer tumours become resistant to aromatase inhibitors," said Dr Magnani, of Imperial's department of surgery and cancer.
"The treatments work by cutting off the tumour's fuel supply - oestrogen - but the cancer adapts to this by making its own fuel supply."
The team, who analysed tumour samples taken from 150 women treated at the European Institute of Oncology in Milan, are working on a test to identify if a tumour has started to make its own oestrogen.
Dr Magnani said that, in the meantime, doctors should take a second sample of the tumour when the cancer returns.
