A multidisciplinary team of scientists led by Seattle based private practice researcher Charles Harding, AB, have recently released results from a nationwide mammography study that found an increased rate of the screening procedure led to a higher rate of diagnosed small breast cancers (≤ 2 cm) without an associated decrease in breast cancer (BC) mortality. The study, entitled, “Breast Cancer Screening, Incidence, and Mortality Across US Counties,” was published in the latest edition of JAMA Internal Medicine.
- Overdiagnosis: the diagnosis of a condition or disease more often than it is actually present.
- Ecological study: is a study in which group data rather than individual data are compared.
- Ecological bias: also known as ecological fallacy- it is a bias that may occur in ecological studies in which assumptions made from analyzing group data are inaccurately interpreted to the individual level.
The researchers conducted an ecological study in which they utilized reported patient information from the year 2000 Surveillance, Epidemiology, and End Results cancer registries and analyzed the data of 53 207 women 40 years or older who resided in 547 counties across the US, and were diagnosed with breast cancer that year and followed up for the next 10 years; to examine the associations between rates of modern screening mammography and the incidence and mortality rates of BC, as well as, tumor size.
The primary study finding showed that throughout the US an increase in the number of screening mammographies conducted was associated with an increase in BC incidence but not in an overall decrease of mortality from the disease. Other important study findings included:
- An increase of 10 percentage points in the number of screening procedures was accompanied by 16% more BC diagnoses but no significant change in BC deaths.
- An increase in screening was strongly associated with an increased incidence of small BC tumors (≤2 cm) but not with a decreased incidence of larger BC tumors (>2 cm).
- An increase of 10 percentage points in screening was associated with a 25% increase in the incidence of small BC tumors and a 7% increase in the incidence of larger BC tumors.
When explaining the significance of the study’s findings the authors wrote, “When analyzed at the county level, the clearest result of mammography screening is the diagnosis of additional small cancers. Furthermore, there is no concomitant decline in the detection of larger cancers, which might explain the absence of any significant difference in the overall rate of death from the disease. Together, these findings suggest widespread overdiagnosis.”
This study adds to the growing body of knowledge that is focused on understanding how the screening guidelines provided to healthcare providers effects disease and mortality rates.
In an accompanying JAMA editorial about the study Dr.s Joann Elmore and Ruth Etzioni of the University of Washington in Seattle agree that the study’s results are limited by the potential of ecological bias, and write, “Overdiagnosis is the diagnosis of a tumor that would not have become clinically apparent in the absence of screening. Treatment of an overdiagnosed tumor cannot provide benefit, but it can lead to harm. Overdiagnosis and overtreatment are now widely acknowledged to be an important harm of medical practice, including cancer screening.”
“Whereas most previous studies have attempted to estimate breast cancer overdiagnosis from trends in disease incidence at a larger population level, this study uses smaller study units and examines patterns at the county level. As with the ecological analysis of mortality data, some caution is required when interpreting the county-level patterns of incidence. Increased screening generally leads to increased incidence, but determining how patterns of smaller and larger tumors might be expected to change under increased screening is quite complex. While observations by Harding et al could have several causes and cannot be unequivocally attributed to overdiagnosis, they add to a growing body of literature on the topic.”
The authors conclude that the findings could leave patients open to confusion on how they should proceed in the future, and that more studies need to be conducted in an attempt to use evidence as a way of eliminating that confusion.