The power of the drug tamoxifen to reduce breast density is key in preventing breast cancer - according to a presentation in America by Cancer Research UK scientists.
Researchers monitored the extent to which tamoxifen could reduce breast density in 7000 healthy post menopausal women who are at high risk of breast cancer in a trial called IBIS 1.
They found that mammograms showed at least a 10 per cent reduction in breast density after 12-18 months of follow-up in 46 per cent of women on tamoxifen. In this group there was a 63 per cent reduction in the risk of developing cancer compared with women who were not given tamoxifen.
But for the rest of the women - where tamoxifen did not reduce breast density - risk of the disease was not significantly reduced.
Professor Jack Cuzick, Cancer Research UK epidemiologist who was presenting the trial results at a breast cancer conference in San Antonio, said: "It is important to find a way to predict who will respond to tamoxifen and changes in breast density appear to be an early indicator of benefit for its use in prevention. Further research is needed to see if these results also apply to the use of tamoxifen for treating breast cancer.
"We know that increased breast density is one of the leading risk factors for breast cancer and where tamoxifen is shown to decrease the density the risk of cancer decreases."
Earlier IBIS 1 results have shown that tamoxifen reduces the risk of oestrogen receptor positive breast cancer(the most common kind) by between 30 to 40 per cent in women at high risk of the disease.
These new results indicate that the benefit only occurs in women who lose breast density and in these women the benefit is almost twice as great.
Researchers at the Cancer Research UK Cambridge Research Institute recently discovered the mechanism by which tamoxifen operates in breast cancer patients and how some women build up resistance to the drug.
Dr Lesley Walker, Cancer Research UK's director of cancer information, said: "Tamoxifen has been a huge success story helping to prevent breast cancer recurring for many women. Understanding why it occasionally stops working is really important because it allows us to identify new targets for drug development and who will need such treatments.
"We are moving into an era of personalised medicine and now, Professor's Cuzick's work has increased our knowledge of which high risk women will benefit from tamoxifen as a preventative drug, and which women won't benefit. This is a key advance as we try to ensure women get the most suitable treatment."