Benefits of mammography may be statistically overestimated
The benefits of mammographic screening are likely to have been overestimated, according to an in-depth review of randomized trials on screening for breast, colorectal, cervical, prostate, and lung cancers. The review was published in the Journal of the Royal Society of Medicine (2015; doi: 10.1177/0141076815593403).
This overestimation results from the use of an unconventional statistical method which differs from that used for other cancer screening trials, concluded the paper. It was co-authored by researchers at King's College London in the United Kingdom and the University of Strathclyde Institute of Global Public Health at International Prevention Research Institute (iPRI) in France.
Starting in the 1960s and 1970s, Swedish randomized trials suggested that mammography screening could reduce breast cancer mortality by 20% to 25% in populations where screening is widespread. These findings were, and remain, extremely influential in decisions taken to establish population breast cancer screening programs using mammography.
The main goal of cancer screening is reducing mortality from the disease by finding cancers at an early stage. Early detection reduces the risk of being diagnosed with an advanced cancer that is often deadly.
In 2002, the World Health Organization recommended that when population screening for breast cancer was implemented in any region, the rate of advanced breast cancers should be monitored. If the program is successful, these rates should fall over time, indicating that mammography screening is contributing effectively to reducing breast cancer mortality. Moreover, with increased screening, more rapid and more pronounced falls in breast cancer mortality would be expected in countries that implemented mammography screening program at end of the 1980s than in countries that implemented programs 10 to 15 years later.
"Contrary to expectations, numerous studies in North America, Europe, and Australia have shown that the rates of advanced breast cancer have not declined in countries where most women regularly attend mammography screening," observed led author Professor Philippe Autier, MD, MPH, from University of Strathclyde Institute of Global Public Health at iPRI.
"Other studies have shown that declines in breast cancer mortality were the same in countries that implemented mammography screening at the end of the 1980s as those that did so 10 to 20 years later. The absence of differences in mortality reductions could not be explained by differences in access to modern therapies," said Autier.
Richard Sullivan, MD, PhD, of the Institute of Cancer Policy and King's College in London observed, "These findings were in sharp contrast with screening for cervical and colorectal cancers, two cancers for which studies have clearly shown the capacity of screening to reduce the numbers of advanced cancers in populations. This has major implications for policy-makers in middle income countries who are now making decisions about where to prioritize cancer screening efforts".
These discrepancies led Autier and his colleagues to undertake an in-depth review of all randomized trials of cancer screening.
"If the Swedish trials had used similar statistical analyses to other cancer screening trials, reductions in the risk of breast cancer death associated with mammography screening would have been much smaller, probably less than 10%," concluded Autier. However, the impact of mammography screening cannot be wholly accounted for without also considering major improvements in care for breast cancer.