According to the results of a recent study, routine breast cancer screening could be incorrectly picking up women who do not actually have the disease.

Mammography screening was linked with higher diagnosis of small cancers but not with meaningful changes in breast cancer deaths, or a decreased incidence of larger breast cancers, in the study.

The study comes amid growing concerns that screening can unintentionally lead to overdiagnosis through identifying small, indolent or regressive tumors that would not otherwise become clinically apparent.

The purpose of screening mammography is to lower breast cancer death by finding and treating cancer early in the course of the disease. If screening detects tumors early, the diagnosis of smaller and more treatable cancers should increase, while the diagnosis of larger and less treatable cancers should decrease.

“Across U.S. counties, the data show that the extent of screening mammography is indeed associated with an increased incidence of small cancers but not with decreased incidence of larger cancers or significant differences in mortality. … What explains the observed data? 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,” the authors write.

During the year 2000, 16 million women ages 40 and older who lived in 547 counties, reported to Surveillance, Epidemiology and End Results cancer registries. Out of these women, 53,207 were diagnosed with breast cancer that year and were followed up for the next 10 years.

Richard Wilson, D.Phil., of Harvard University, and the study co-authors looked at the extent of screening in each county and measured breast cancer incidence in 2000 and incidence-based breast cancer death during the 10-year follow-up, with incidence and mortality calculated for each county.

An increase of 10 percentage points in the extent of screening was associated with 16 percent more breast cancer diagnoses but not significant change in breast cancer deaths.

“As is the case with screening in general, the balance of benefits and harms is likely to be most favorable 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 active treatment,” the study concluded.

Charles Harding, AB; Francesco Pompei, PhD; Dmitriy Burmistrov, PhD; H. Gilbert Welch, MD, MPH; Rediet Abebe, MASt; Richard Wilson, DPhil Breast Cancer Screening, Incidence, and Mortality Across US Counties JAMA Intern Med. Published online July 06, 2015. doi:10.1001/jamainternmed.2015.3043

Illustration: Wellcome Images, Creative Commons by-nc-nd 4.0

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