EPIDEMIOLOGIC CORRELATES OF BREAST-CANCER LATERALITY (SWEDEN)

Breast cancer laterality was studied in relation to age in 80,784 case s of invasive and 3,835 cases of pre-invasive breast cancer in women a nd 548 cases of invasive breast cancer in men reported to the Swedish Cancer Registry, 1970-89. In a subset of 11,274 women with invasive di sease, data on parity were available through the Swedish Fertility Reg istry. Laterality also was evaluated in relation to age and reproducti ve variables in 3,986 cases from an international study from the 1960s . The overall incidence of pre-invasive and invasive cancer was higher in the left than in the right breast among both women and men. The ex cess incidence of invasive cancer in the left breast was evident only after the age of 45 years in women; a similar phenomenon may exist wit h pre-invasive disease in women and in men. The age-dependent laterali ty pattern did not appear to be confounded by menopausal status. Among women younger than 45 years, nulliparity, right handedness, and late age at menarche was associated with a somewhat higher incidence of can cer in the right breast. The laterality findings are likely to be due to factors operating early in the carcinogenic process, perhaps at the pre-initiation stage.