Epidemiology
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It is well established that the incidence rates of first primary breast cancer have been increasing over time. In contrast, the incidence rates of second primary breast cancer are largely undocumented. This study describes the epidemiology of second primary breast cancer among a population-based cohort of 305,533 U.S. women diagnosed with breast cancer between 1973 and 1998. ⋯ The pattern of incidence rates for first and second primary breast cancer differ markedly over time and by age.