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BREAST cancer can return 20 years after a woman is given the all-clear, a study reveals.
The disease can ‘lie dormant', Oxford University researchers found.
It means women might be told to continue taking hormonal drugs for longer than the current five years, in a bid to stop tumours returning.
Scientists analysed data from 88 clinical trials involving 62,923 women, all of whom had the most common form of breast cancer fuelled by the hormone oestrogen.
Every patient received pill treatments such as tamoxifen or aromatase inhibitors which block the effects of oestrogen or shut off the hormone's supply. After five years of therapy, their cancers had gone and they stopped taking the drugs.
But monitoring the women's progress revealed recurrences of the disease up to 15 years later, 20 years after initial diagnosis. It is the most common cancer among British women, with more than 55,000 diagnosed a year.
Survival rates have significantly improved in recent years. 
In the past most people with cancer were likely to die within a few years, but medical advances mean patients are more likely to survive for many years – which is why scientists are learning for the first time that tumours can come back so long afterwards.
Lead researcher Dr Hongchao Pan, from Oxford University, said: ‘It is remarkable that breast cancer can remain dormant for so long and then spread many years later, with this risk remaining the same year after year and still strongly related to the size of the original cancer and whether it had spread to the lymph nodes.' 
Women who started off with large tumours and cancer that had spread to four or more lymph nodes faced the highest risk of recurrence, the study showed. 
They had a 40 per cent risk of cancer returning in a different part of the body over a period of 15 years after stopping treatment.
For patients diagnosed with small, low-grade cancers that had not spread the risk was 10 per cent.
Doctors have long known that five years of tamoxifen reduces the risk of recurrence by about a third in the five years after stopping treatment.
Aromatase inhibitors, which only work for post-menopausal women, are believed to be even more effective.
But Sally Greenbrook, Policy Manager at Breast Cancer Now, hailed the ‘important development'.
She said: ‘We've always known that breast cancer can return years later, but this major study identifies that women may remain at risk of recurrence for at least 15 years, suggesting that they may benefit from extending their hormone therapy. 
‘As women taking hormone therapies can experience difficult side effects, it's essential they discuss any changes in treatment with their doctor to make a decision that's right for them.'

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