While it makes sense that detecting a second tumor early improves the chances of survival, until now there has been little evidence that it does, the researchers said, adding that this had implications for monitoring of patients.

The findings published in the Annals of Oncology underline the importance of follow-up treatment and close monitoring for women who have survived a first tumor, Nehmat Houssami of the University of Sydney in Australia and colleagues reported.

"Intuitively, it makes sense to consider that early detection of second breast cancers will improve prognosis, since breast cancer survivors have a long-term risk of developing further disease or relapse in either breast," she said in a statement.

"However, due to a paucity of evidence about this until now, current recommendations on surveillance of breast cancer survivors vary substantially between countries and organizations."

Breast cancer is the leading cancer killer of women worldwide, according to the American Cancer Society. The group estimates about 465,000 women died from breast cancer globally in 2007, with 1.3 million new cases diagnosed.

Early detection through mammography screening and improved treatments has driven declines in breast cancer deaths in developed countries, it says.

The researchers looked at 1,044 women who attended a clinic in Italy between 1980 and 2005 after developing a second breast tumor.

They found that after adjusting for the fact some tumors grow more slowly than others, a woman's chances of survival were improved by between 27 percent and 47 percent compared to women whose cancers were detected at a later stage when symptoms started to appear.