Evidence-Based Answers

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Word of the Day

Overdiagnosis of breast cancer is common

Clinical Question:
What proportion of breast cancer is overdiagnosed?

Bottom Line:
In the past 30 years we have seen a large increase in the detection of early-stage cancers, but little corresponding decline in late-stage cancers. The authors conclude that approximately 1 in 4 breast cancers has been overdiagnosed, and was unlikely to have ever harmed the woman. (LOE = 2b)

Bleyer A, Welch HG. Effect of three decades of screening mammography on breast-cancer incidence. N Engl J Med 2012;367(12):1998-2005.  [PMID:23171096]

Study Design:
Cohort (prospective)

Self-funded or unfunded


As our understanding of cancer biology has evolved, we now recognize that some cancers detected by screening never would have harmed the patient. These cancers are "overdiagnosed" and are a significant source of harm, since these patients face the burden of treatment, its adverse effects, and the psychological harm of living with a cancer diagnosis. If a screening program is effective, one would expect to see more early-stage cancers detected and (important!) fewer late-stage cancers detected over time. In this study of breast cancer screening, early stage was defined as carcinoma in-situ or localized invasive cancer, and late stage was defined as any cancer with regional or metastatic spread at the time of diagnosis. The authors first determined the baseline incidence of early- and late-stage breast cancer using data from 1976 to 1978, when the use of screening mammography was uncommon. They eliminated the effect of hormone replacement therapy (which increased the incidence of breast cancer) by truncating the incidence between 1990 and 2005 at the levels for 2006 to 2008. For the years from 1978 to 2008, they multiplied the increase or decrease of early- and late-stage cancers in a given year by the number of women 40 years and older during that year. This gave them the excess number of early-stage cancers and the reduction in late-stage cancers, summed over the years from 1978 to 2008. They found a large increase in early-stage cancers (from 112 to 234 per 100,000 women per year), but only a small decrease in regional or distant disease (from 102 to 94 cases per 100,000 per year). On the other hand, in women younger than 40 who typically do not undergo screening mammography, there was no change in the number of early- or late-stage cancers. The authors assumed that there was a small increase in the annual risk of breast cancer, similar to that seen in younger women. Their conclusion is that more than 1.3 million women have been overdiagnosed in the past 30 years (approximately 25% of all breast cancers). Varying the assumptions provided a range of 22% to 31% overdiagnosed breast cancers and 50,000 to 70,000 overdiagnosed women per year.


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