A new large-scale study of women with breast cancer finds a 25% increase in their risk of developing a new non-breast cancer, compared to women who are cancer-free. The researchers analyzed records of over 500,000 breast cancer patients in Europe, Canada, Australia and Singapore.

Lene Mellemkjaer is a senior scientist with the Danish Cancer Society and lead author of the study. She says the data give the most precise estimate to date of what's known as secondary cancer risk. "We also estimated the absolute number of extra cases and this 25% increase in risk it corresponds to 16 or 17 extra cancer cases per 10,000 women with breast cancer per year," she says.

The analysis does not suggest what might be causing these extra cases, although other studies indicate that cancer treatment itself could be a factor. "The next step would be to try to estimate how important is the treatment and other risk factors," she says and adds, "It would be relevant to try to identify some characteristics of breast cancer patients that could be indicators of a susceptibility to development of a second cancer."

But Lene Mellemkjaer says women should not refuse treatment for fear of developing a second cancer. That is because the risk of having second breast cancer is relatively small and the treatment is crucial for the survival after breast cancer.

The study was published online in the December issue of the International Journal of Cancer.