Mammography detects more small cancers but results in overdiagnosis, researchers warnBMJ 2015; 351 doi: https://doi.org/10.1136/bmj.h3661 (Published 07 July 2015) Cite this as: BMJ 2015;351:h3661
Screening mammography results in overdiagnosis, researchers have warned, as more small cancers are detected but there is no corresponding decline in the detection of larger cancers and no significant change in deaths from breast cancer.
An ecological study published in JAMA Internal Medicine1 examined cancer registry data on 16 million women aged 40 and over who lived in 547 US counties in the year 2000. Of these women, 53 207 had breast cancer diagnosed that year and were followed up for the next 10 years.
Across the counties a positive correlation was found between the extent of screening and breast cancer incidence but not with breast cancer mortality. An absolute increase of 10 percentage points in the extent of screening was associated with 16% more breast cancer diagnoses (relative risk 1.16 (95% confidence interval 1.13 to 1.19)) but no significant change in breast cancer deaths (1.01 (0.96 to 1.06)).
More screening was strongly associated with a higher incidence of small breast cancers of 2 cm or less but not with a lower incidence of larger breast cancers over 2 cm. An increase of 10 percentage points in screening was associated with a 25% increase in the incidence of small breast cancers and a seven percentage point increase in the incidence of larger breast cancers.
Richard Wilson, of Harvard University in Cambridge, Massachusetts, and coauthors said, “The simplest explanation is widespread overdiagnosis, which increases the incidence of small cancers without changing mortality, and therefore matches every feature of the observed data.”
But they did not believe that the right rate of screening mammography was zero, they said, adding, “Instead the balance of benefits and harms is likely to be most favourable when screening is directed to those at high risk, provided neither too frequently nor too rarely, and sometimes followed by watchful waiting instead of immediate treatment.”
Cite this as: BMJ 2015;351:h3661
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