Reproductive factors and breast cancer risk. Do they differ according to age at diagnosis?

Studies yielding results on risk factors stratified by age at diagnosis or menopausal status were reviewed to better understand the role of hormonal factors and to determine whether reproductive events influence breast cancer risk differently according to age at diagnosis of breast cancer. Through a Medline/Pubmed search, 26 articles providing risk estimates by age at diagnosis of breast cancer or by menopausal status were analysed. A decrease of about 9% of breast cancer risk was found for each additional year at menarche when breast cancer was diagnosed early or before the menopause, and of about 4% when diagnosed late or after. Breast cancer risk increased with increasing age at FFTP by 5% per year for breast cancer diagnosed early or before the menopause and by 3% for cancers diagnosed late or after the menopause. Each full term pregnancy (or child) led to a 3% reduction in the risk of breast cancer diagnosed early or before the menopause, whereas the reduction attained 12% for the breast cancers diagnosed later. No change in the effect of these three factors with time (date of diagnosis of the breast cancer before 1980 or after) was observed. These results support the hypothesis of an age-specific effect of the three breast cancer risk factors considered herein, based on the time of initiation of the carcinogenic process. These observations underline the importance of the time of initiation of the carcinogenic process in determining the effect of promoters such as reproductive factors. This largely unexplored aspect of breast carcinogenesis might open the way for new prevention approaches.