Researchers have revealed through a new study that screening mammography is effectively an over-diagnosis and that there are no proofs that such a step has helped bring down the number of cancer deaths.
The primary reason why screening mammography is used is to reduce breast cancer death by detecting and treating cancer early in the course of the disease. If through screening mammography tumors are detected early, then the diagnosis of smaller and more treatable cancers should increase, while the diagnosis of larger and less treatable cancers should decrease. However, there are increasing concerns that screening unintentionally leads to overdiagnosis by identifying small, indolent or regressive tumors that would not otherwise become clinically apparent.
In an article published online by JAMA Internal Medicine, though screening mammography is associated with increased diagnosis of small cancers, there is no proof that there have been significant changes in breast cancer deaths or a decreased incidence of larger breast cancers.
For the latest study, researchers conducted an ecological study of 16 million women ages 40 and older who lived in 547 counties reporting to Surveillance, Epidemiology and End Results cancer registries during the year 2000. Of these women, 53,207 were diagnosed with breast cancer that year and followed up for the next 10 years.
The authors examined 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.
The authors found that across counties there was a correlation between the extent of screening and breast cancer incidence but not with breast cancer mortality. 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.
More screening also was associated with increased incidence of small breast cancers of 2 centimeters or less but not with a decreased incidence of larger breast cancers, according to the results. An increase of 10 percentage points in screening was associated with a 25 percent increase in the incidence of small breast cancers and a 7 percent increase in the incidence of larger breast cancers.
“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 conclude.
However, the authors note clinicians are correct to be wary of ecological studies because of ecological fallacy, which is making inferences about individuals from group data in statistical analyses because individuals may not possess those characteristics.
“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 concludes.
