While mammography was associated with a 16% increase in breast cancer diagnosis, researchers found it was not linked with a reduction in deaths from the disease.
After skin cancer, breast cancer is the most common cancer among American women. Around 231,840 women in the US will be diagnosed with breast cancer this year, and around 40,290 will die from the disease.
As with other cancers, early detection of breast cancer is key for successful treatment, and this can be achieved through breast cancer screening. The main tool used for breast cancer screening is mammography, which involves taking an X-ray of each breast, allowing clinicians to see any tissue abnormalities.
The US Preventive Services Task Force (USPSTF) recommend that women aged 50-74 have a mammogram every 2 years.
According to the American Cancer Society, death rates from breast cancer have been falling since around 1989, and this is partly attributed to earlier detection as a result of breast cancer screening.
However, study co-author Richard Wilson, of Harvard University in Cambridge, MA, and colleagues note that there is increasing concern that mammography may lead to overdiagnosis by "identifying small, indolent or regressive tumors that would not otherwise become clinically apparent," which means many women may receive treatment they do not necessarily need.
What is more, although clinical trials have shown mammography is effective for early breast cancer diagnosis, Wilson and colleagues note that most of these trials are decades old. "There are concerns that the benefits and harms may have changed as treatments improved and screening was applied in general practice," they add.
Breast cancer screening increased diagnosis by 16%, but did not reduce death rates
For their study, the team set out to assess the link between rates of mammography for breast cancer detection and breast cancer incidence, tumor size and death rates from the disease.
They analyzed data from the Surveillance, Epidemiology and End Results (SEER) cancer registries, involving more than 16 million women aged 40 and older from 547 counties in the US.
Breast cancer was diagnosed in 53,207 of these women during the 12-month period, and these women were followed-up over the next 10 years.
The rate of breast cancer screening was assessed in each county, as determined by the percentage of women who underwent a mammogram in the previous 2 years.
Overall breast cancer incidence in the year 2000 was calculated for each county, as was the rate of breast cancer deaths during the 10-year follow-up. The team age-adjusted this data and applied it to the US population.
The results of the analysis revealed a 10% rise in breast cancer screening. This was associated with a 16% increase in breast cancer diagnosis. However, no reduction was found in the rate of breast cancer deaths.
In addition, the 10% increase in breast cancer screening was linked to a 25% rise in incidence of small breast cancers - defined as the presence of tumors 2 cm or less. However, the increase in breast cancer screening was not associated with a reduction in incidence of larger breast cancers - it was linked to a 7% increase.
Commenting on their findings, Wilson and colleagues say:
"Across US 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."
Clinicians 'right to be wary' of breast cancer screening studies
The researchers add, however, that clinicians are right to have concerns about ecological studies regarding breast cancer screening because of "ecological fallacy." Dr. Joann G. Elmore, of the University of Washington, agrees with this statement in a linked editorial.
"It is well known, for example, that ecological studies provide no information as to whether the people who were actually exposed to the intervention were the same people who developed the disease, whether the exposure or the onset of disease came first, or whether there are other explanations for the observed association," she explains.
As such, she says better tools and communication are required to help women make informed decisions about breast cancer screening.
"Perhaps most important, we need to learn how to communicate with our patients about uncertainty and the limits of our scientific knowledge," she adds. "In the end, we all need to become comfortable with informing women that we do not know the actual magnitude of overdiagnosis with precision. Part of informed decision making is providing all the information, even our uncertainty."
In contrary to these latest findings, a study reported by Medical News Today last month claims mammography is the best screening method for reducing breast cancer mortality among women aged 50 and older.