Most women with breast cancer have at least 1 risk factor usually recognized at the time of mammography, according to a case-control study involving more than 200,000 women. More than half of breast cancers are explained by these risk factors.

Participants included women with invasive breast cancer or ductal carcinoma in situ (n=18,437) and women without breast cancer (n=184,309). ~70% were postmenopausal. Investigators collected risk factor data prospectively between 1996 and 2012. Among the results:

• 90% of premenopausal and 95% of postmenopausal women with breast cancer had at least 1 risk factor.
• The combined population-attributable risk proportion of all risk factors was 53% in premenopausal and 55% in postmenopausal women.
• Breast density was the most prevalent risk factor.
• In postmenopausal women, 23% of breast cancers could be averted if all overweight and obese women attained a body mass index of <25.