Between 1976 and 2008, a total of 1.3 million women in the United States were overdiagnosed, the researchers estimate.
The overdiagnosis rate was similar for all age groups over 40, although it was slightly higher among women between 50 and 70 years, who may be the ones most likely to have routine mammograms, Bleyer said. In contrast, the incidence of both early- and late-stage breast cancer was largely unchanged between 1976 and 2008 among women under 40 years, who do not receive mammogram screening.
Although Bleyer was surprised to discover that the United States had such a high overdiagnosis rate among women 40 years and older, "we are not the only one," he said. Studies of Scandinavian populations estimated rates of breast cancer overdiagnosis between about 20 and 30 percent, while an Australian study put their rate at between 30 and 42 percent.
In addition to the questions that the current study raises about the ability of mammography to reduce breast cancer disease, mammography has its own downsides, such as causing anxiety and discomfort for women, Bleyer said. For women who are overdiagnosed, mammography can also lead to unnecessary treatment, such as surgery, chemo and radiation therapy.
To learn more about breast cancer risk factors, visit the U.S. Centers for Disease Control and Prevention.
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