Principles of Cancer Screening
The decision to screen an asymptomatic population for preclinical disease is based on well-established, although not specific, criteria that relate to the disease in question and the characteristics of applicable screening test(s). The first goal of screening for breast cancer on an individual or population basis is to distinguish among those who are likely and not likely to have the disease. The emphasis on likelihood underscores the limits of what can be expected of screening (i.e., screening is not a diagnostic process).
It is inherent to the concept of screening that a person identified with an abnormality is then referred for further diagnostic testing to determine whether the disease truly is present. Further, the emphasis on likelihood also is important because screening programs have inherent limitations that are a function of the specific disease, differences between individuals, available technology, and finally, the basic criteria for the acceptability of a screening test - in other words, its ability to efficiently test a large asymptomatic population at an acceptable cost.
In the case of breast cancer screening, the majority of screening interpretations are accurate, but it is inevitable that some women will be identified as possibly having breast cancer when they do not, and screening tests will fail to identify some women who do have the disease.
Before large numbers of asymptomatic individuals undergo routine testing for the presence of a chronic disease, certain conventional criteria should be met. First, the disease should be an important health problem. Second, there should be a period when the disease is detectable in an asymptomatic individual (i.e., a detectable preclinical phase).
Treatment at this early stage should offer better outcomes than if the disease were treated at a later stage. Third, the screening test must be effective, reasonably accurate, and affordable or at least not prohibitively expensive. With respect to accuracy, ideally the test should achieve acceptable levels of sensitivity and specificity.
The importance of accuracy is unquestionable, but the comparative consequence of a false-positive or false-negative test result may also be a consideration when determining thresholds of acceptability. Moreover, the complements of sensitivity and specificity (i.e., false-negatives and false-positives) provide insights not only into what is being achieved in a screening program but also how accuracy can be improved and, thus, what potentially is achievable in a screening program.
Finally, the test must be acceptable to targeted individuals and referring physicians.
Screening for breast cancer meets each of these criteria well enough, although current data indicate that screening may not be equally effective among all age groups, an observation that has been a source of ongoing health policy debate.
The controversy over the value of screening women between the ages of 40 to 49 is well known, but there are also differences of opinion about the benefit of screening women older than age 70 and different ideas about recommended screening intervals for women of different ages. There are also concerns about various costs associated with screening.
These costs, both financial and psychosocial, include the proportion of women who receive false-positive results, the anxiety experienced by women whose mammograms are interpreted as abnormal and those who undergo avoidable biopsy, and the rate of diagnosis of ductal carcinomas in situ (DCIS), some of which would not eventually progress to invasive disease.
These costs are regarded by some as too high, by others as acceptable or at least unavoidable, and by others as evidence of a need to strive for better performance. We address each of the previously mentioned criteria as a basis for summarizing what we know about the effectiveness of screening, as well as key issues related to the status of breast cancer screening in clinical practice today.
Robert A. Smith and Carl J. D’Orsi
R. A. Smith: Cancer Screening, Department of Cancer Control, American Cancer Society, Atlanta, Georgia
C. J. D’Orsi: Diagnostic Radiology, University of Massachusetts Memorial Medical Center, Worchester, Massachusetts