Breast cancer: geographic variation and risk factors.

Cancer of the breast is the most frequent malignant neoplasm occurring in women in developed countries. The age-standardized incidence rates of breast cancer vary widely throughout the world, the highest being observed in white females in the U.S. In general, its incidence is much lower in developing countries, particularly in Asia. Incidence rates in Japan are also low, which contradicts the role of the high economic level of a country being associated with high risk of breast cancer. The range of variation in incidence worldwide reaches a tenfold difference. Though breast cancer incidence has been increasing, screening practices for early diagnosis have contributed to keeping mortality at lower levels. Geographical variations, time trends, and studies on populations migrating from low- to high-risk areas, which show that the incidence in such populations approaches that of the host country in one or two generations, clearly suggest an important role of environmental factors in the etiology of breast cancer. Since the 1960s, numerous analytical studies have been conducted that have investigated, in particular, reproductive habits, hormonal patterns, and more recently, dietary habits. This article reviews the known risk factors and current research directions.