Risk factors for early age at breast cancer onset--the "COSA program" population-based study.

BACKGROUND Breast cancer is the most frequent cancer in women in western countries. A number of risk factors are now known, but the etiology of the disease is not fully understood. The aim of this study was to determine the effects of reproductive, anthropometric and environmental factors on cancer onset. PATIENTS AND METHODS 934 women who developed a non-hereditary breast cancer were recruited from different hospitals in the Auvergne region (France) and completed a questionnaire. RESULTS The use of oral contraceptives (OC) increased the risk of early cancer development (odds ratio = 1.84, 95% confidence interval = 1.38-2.44). The age at first OC use appeared to be a major factor since the risk decreased when OC use was after the age of 23 years (odds ratio = 0.52, 95% confidence interval = 0.34-0.79). A duration of breast-feeding greater than 26 weeks decreased the risk of early cancer development (odds ratio = 0.62, 95% confidence interval = 0.39-0.97). No overall association was found with anthropometric or lifestyle factors and early age at breast cancer onset. CONCLUSION OC use, age at first OC use and lactation were significantly associated with an early age at breast cancer onset. Thus, a number of "risk factors" could be considered as "early onset risk factors".