RESUMO
BACKGROUND: Younger age at menarche and older age at first live birth are implicated as risk factors for breast cancer, but the extent to which these factors contribute to the sharply rising incidence of this disease in developing countries has received little study. METHODS: We conducted a retrospective analysis of 702 consecutive breast cancer patients diagnosed at a single hospital in Hong Kong during 2003-2006. Comparisons were made between patients with different ages at cancer diagnosis (hence, belonging to different birth cohorts) and their respective ages at menarche and at first live birth. We then correlated these age-dependent differences with overall breast cancer incidence data from the Hong Kong Cancer Registry. RESULTS: When patients diagnosed before age 40 are compared with those after 60, the age of menarche is lower in the former subgroup (12.7 vs. 14.2; p<10(-6)) while the age of first live birth is greater (28.2 vs. 25.5; p<0.01). However, registry data suggest that the progressive rise in breast cancer incidence has not affected those over 65, nor very young (20-39 years) patients. CONCLUSION: Lifestyle variables that reduce age at menarche may contribute to the rising risk of breast cancer diagnosed after age 40 in Hong Kong, whereas earlier-onset cancers may be characterised by a distinct pathogenesis. Although retrospective, these data raise the possibility that cancer-preventive health interventions could gainfully target reversible risk factors favoring early menarche - such as formula infant feeding, high-fat diets, and lack of exercise - in children and adolescents living in developing countries such as China.