Risk factors for breast cancer according to age at diagnosis in a French case-control study.

In a French case-control study of 1010 women with breast cancer and 1950 controls with nonmalignant disease, the variations of the effects of 8 risk factors for breast cancer as a function of age at diagnosis, were analysed by tests of homogeneity and trend. The risks associated with a late age at first full-term pregnancy and with nulliparity were different between age-groups (test of homogeneity: p = 0.03), and the highest risks for these two factors were observed in women 45-54 years old. The risks associated with Quetelet index were also found to vary with age at diagnosis (test for trend: p = 0.008). A high Quetelet index decreased the risk of breast cancer in the younger age-groups; this decrease of risk became progressively less important with advancing age, and no such effect was found in the oldest age-group. Inverse results were observed for a tall stature (test for trend: p = 0.04): a tall stature increased the risk of breast cancer in the younger age-groups, and the figures suggested a reverse effect in the oldest group. No large variation with age was found for the effects of age at menarche, history of breast cancer death in mother or sisters, prior biopsy for benign breast disease, and weight. In conclusion, the relative importance of certain risk factors for breast cancer is closely related to age at diagnosis. Nulliparity and a late age at first birth appear to be major risk factors only for middle-aged women, whereas a low Quetelet index and a tall stature appear to increase the risk of breast cancer only for younger women.