Study confirms risks and benefits of long-term tamoxifen treatment
New US research has confirmed that women who take the hormone therapy tamoxifen to reduce the risk of their breast cancer recurring are less likely to develop the most common type of breast cancer, but may face an increased risk of developing a more aggressive form of the disease.
However, Cancer Research UK said that the benefits of taking tamoxifen after a first diagnosis of breast cancer "far outweigh any potential risks".
The finding comes from new research by scientists at the Fred Hutchinson Cancer Research Centre in the US, who studied the effect of tamoxifen in 1,103 breast cancer survivors.
Tamoxifen is an oestrogen-blocking drug that is usually prescribed to women with oestrogen receptor (ER) positive breast cancer.
The drug prevents oestrogen from driving breast cancer growth.
However, the latest results, which appear in the journal Cancer Research, build on research published by the team in 2001 that suggested a link between long-term tamoxifen use and an increased risk of more aggressive, ER-negative second cancers.
The researchers compared the medical histories of 369 ER-positive breast cancer survivors whose cancer had recurred, and compared these with the records of 728 breast cancer survivors who were similar in all respects except they had not had a recurrence of their disease.
The researchers found that women who had been given tamoxifen for over five years were 40 per cent less likely to have a second ER-positive breast cancer in their opposite, or 'contralateral', breast.
However, women who took the drug for five or more years were four times more likely to develop a second, ER-negative, cancer - although the actual number of women who developed these cancers was small.
Using the drug for less than five years did not increase this risk.
Dr Christopher Li, who was involved in both studies, said: "This is of concern, given the poorer prognosis of ER-negative tumours, which are also more difficult to treat.
"It is clear that oestrogen-blocking drugs like tamoxifen have important clinical benefits and have led to major improvements in breast cancer survival rates," the scientist noted.
"However, these therapies have risks, and an increased risk of ER-negative second cancer may be one of them. Still, the benefits of this therapy are well established and doctors should continue to recommend hormonal therapy for breast cancer patients who can benefit from it."
Professor Jack Cuzick, head of Cancer Research UK's Centre for Epidemiology, Mathematics and Statistics at Queen Mary, University of London, commented: "There is overwhelming evidence that tamoxifen and newer, more effective hormone-blocking treatments prevent far more recurrences, new breast cancers and cancer-related deaths than they might stimulate. "Work on understanding how tumours eventually escape from hormone treatment will be very valuable. However, the use of hormone-blocking drugs to treat oestrogen receptor positive breast cancer will continue to be the main form of treatment."
Dr Alison Ross, senior science information officer at Cancer Research UK, said: "Women should be reassured that, based on extensive scientific evidence, the benefits of taking hormone-blocking drugs, such as tamoxifen, after their first diagnosis of breast cancer far outweigh any potential risks."
Dr Ross noted that the study confirms the "long-established benefits" of hormone-blocking drugs.
She added: "More research will be needed to confirm the possible link between its long-term use and the relatively rare occurrence of an aggressive form of the disease in the other breast."