breats x-ray
© PA:Press Association/PA ImagesResearchers now believe that drugs given to thousands of breast cancer patients could be creating 'sleeper cells' that later cause incurable tumours
Drugs taken by thousands of breast cancer patients create deadly "sleeper cells" that may cause incurable tumours years later.

Researchers say the hormone therapies kill most of the cancer cells but put some into a dormant state and help them to spread.

These can reawaken up to 20 years later, causing tumours elsewhere in the body that are resistant to treatment.

Scientists from Imperial College London made the discovery while studying 50,000 human breast cancer cells in a lab.

They believe they can use their findings to develop treatments that keep the cells asleep for longer or awaken them so they can be killed.

Hormone therapies are used to treat a type of breast cancer called oestrogen-receptor positive.

They account for 70 per cent of the 55,000 cases of breast cancer in the UK each year and are fuelled by the hormone oestrogen.

These cancers are usually treated with surgery to remove the tumour, followed by aromatase inhibitors or tamoxifen.

Patients see their cancer return

Around 30 per cent of breast cancer patients taking hormone therapies see their cancer return - sometimes up to 20 years later.

This returning cancer is usually metastatic, meaning it has spread around the body. And the tumours are often resistant to drugs.

Dr Luca Magnani, from Imperial, said: "If we can unlock the secrets of these dormant cells, we may be able to find a way of preventing cancer coming back."

Dr Rachel Shaw, from Cancer Research UK, said: "This study highlights a key route researchers can now explore to tackle 'sleeping' cancer cells that can wake up years after treatment.

"It could potentially save the lives of many more women with the disease."

The findings are published in the journal Nature Communications.