Risk factors, lifetime risk, and age at onset of breast cancer

PURPOSE: We evaluated the relationship between exposure variables and both lifetime risk and mean age at diagnosis of breast cancer in subjects from the Adventist Health Study who developed breast cancer before the age of 91 years. METHODS: Multiple decrement life-table analysis was used. This study provided data from 20,341 women followed for 6 years. RESULTS: In the total population, 30-year-old women with a parental history of any cancer or a maternal history of breast cancer had, respectively, 72% (P < 0.002) and 98% (P < 0.03) higher lifetime risks of breast cancer. Thirty-year-old women who had their first delivery after age 24 years or body mass indices above the 50th percentile had, respectively, 53% (P < 0.007) or 57% (P = 0.01) greater lifetime risk of breast cancer. Women who exercised infrequently had a 27% higher life-time risk (P = 0.09) and an age at diagnosis of breast cancer 6.6 years younger (P < 0.005) than other women. CONCLUSIONS: Standard risk factors account for substantial increases in lifetime risk of breast cancer and may be associated with differences in age at diagnosis.