Dense breasts and high levels of sex hormones independently increase the risk of developing breast cancer, new research suggests.
A team from Harvard and Georgetown universities found that post-menopausal woman with dense breasts had a 400 per cent higher relative risk than women with fatty, non-dense breast tissue and that high versus low levels of oestrogen and testosterone increased the relative risk by 200 per cent.
Co-author Dr Celia Byrne said: "We have found that dense breasts are not a marker for higher hormone levels or vice versa and this tells us each increases breast cancer risk via a different biochemical pathway."
She advised women to have a mammogram to determine whether they have dense breasts and then have regular check-ups if they find they do.
Dr Byrne said she was surprised by the findings. "Our assumption was that this study would confirm what everyone had believed; that hormone levels explain breast density."
This assumption was partly based on the fact that during pregnancy breast size increases along with the concentration of sex hormones.
Recent research has intimated that having dense breast tissue may be partially an inherited genetic trait.