A detailed analysis of randomized screening trials, carried out by investigators at King’s College London and the University of Strathclyde (both UK), has suggested that the use of unconventional statistical analyses could mean that the benefits of screening mammography were overestimated in initial studies. Their review appeared online this week in the Journal of the Royal Society of Medicine, adding to the discussion promoted by a Harvard University (MA, USA) study earlier in the week, which indicated widespread overdiagnosis of breast cancer in the US upon screening.
Randomized trials carried out in Sweden beginning in the 1960s and 1970s initially concluded that widespread mammography screening for breast cancer could reduce disease-related mortality by 20–25%, as it facilitates earlier detection of the disease. Since 2002, population screening or breast cancer has been accompanied by monitoring of the rates of advanced breast cancer under WHO recommendations. If screening is successfully detecting breast cancer at an earlier stage, it would be expected that rates of advanced breast cancer should decline, indicating that mammography screening is contributing effectively to reducing breast cancer mortality.
“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 Philippe Autier, lead author from University of Strathclyde. He also described how: “other studies have shown that declines in breast cancer mortality were the same in countries that implemented mammography screening end of the 1980s as those that did so ten to twenty years later. The absence of differences in mortality reductions could not be explained by differences in access to modern therapies.”
Noting that these results deviated from those seen in colorectal and cervical screening, for which evidence clearly indicates a decrease in the occurrence of advanced cancers in the populations, the team carried out a review of all randomized cancer screening trials. Investigating screening studies for breast, colorectal, cervical, prostate and lung cancers, they determined that if the Swedish breast screening trials had utilized similar statistical methods to the other trials investigated, the reductions in risk of breast cancer death owing to screening would have been lower – potentially <10%.
“The reduction seen in the mortality from breast cancer in many countries is one of the major contributions to Cancer Control in recent times,” noted Peter Boyle of the University of Strathclyde. “Many factors have contributed to this success, including earlier presentation and better diagnosis, as well as major improvements in the organization of care (multidisciplinary teams) to specific improvements in surgery, radiotherapy and chemotherapy/endocrine therapy. Currently, assessment of the impact of mammographic screening programmes cannot be made without taking advances in breast cancer treatment into account.”
The authors suggest that these findings could have implications for policy-makers in middle-income countries, potentially influencing decisions about priorities for cancer screening efforts.