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Guidelines for Screening Mammography: The Controversy

Barbara A. DeBuono, MD, MPH, John P. Fulton, PhD

Published in: Rhode Island Medicine, 1994;77(10) 360-361

Barbara A. DeBuono, MD, MPH is Director of Health, State of Rhode Island and Providence Plantations. John P. Fulton, PhD is Assistant Director, Division of Preventive Health Services, Rhode Island Department of Health, and Clinical Assistant Professor of Community Health, Brown University School of Medicine.

Changes

On December 8, 1993, the National Cancer Institute (NCI) released the following statement:

At the same time, the NCI withdrew its standing recommendation for screening mammography every 1 to 2 years among women ages 40 to 49. This position followed the findings of an International Workshop on Breast Cancer Screening, convened by the NCI in February, 1993, and is consistent with the positions of the American College of Physicians and the United States Preventive Services Task Force.

In response, the American Cancer Society (ACS) reaffirmed its standing recommendation for screening mammography every 1 to 2 years among women ages 40 to 49. This recommendation had emerged from a consensus conference of 12 major medical organizations sponsored by the American College of Radiology. The American College of Gynecology also reaffirmed the recommendation of the consensus conference.

The result of NCI's statement has been confusion among women ages 40-49, and concern among primary care physicians. The latter must decide to recommend mammography, per ACS's guidelines, or not, per NCI's position, almost arbitrarily.

How Did This Happen?

In 1969 Shapiro and Strax demonstrated a 30 percent decrease in breast cancer mortality among women ages 40 to 64 who received clinical breast examinations and screening mammograms in the HIP trials (Health Insurance Plan of Greater New York). Since then, the effectiveness of screening mammography has been studied with clinical trials, demonstration projects, and by following cancer rates among women ages 40 and over as screening mammography has become more widespread. Periodically, screening experts have assembled to look at screening results and make recommendations for screening content and frequency. Through 1987, when near consensus was reached in the U.S. on breast cancer screening guidelines, cumulative evidence indicated lower mortality among women ages 40-49 who had received screening mammography. In 1992 results of the Canadian trials were published, showing higher mortality among women ages 40-49 who had received screening mammography. Experts reviewing the evidence in 1993 split in their interpretations of the cumulative results available at that time.

It is important to note that none of the results to date have been statistically significant for women ages 40-49, primarily because none of the studies have been designed with sufficient statistical power to test the effect of screening mammography in this age group. In short, no study enrolled sufficient numbers of women ages 40-49 to make a definitive statistical statement on this subject. The Canadian study, purportedly focused on women ages 40-49, also suffered from insufficient statistical power.

Why the Controversy?

The experts split in 1993 because they looked at different evidence. Those who formulated the ACS position weighed all available medical evidence. Those who formulated the NCI position restricted themselves to evaluating the results of clinical trials, and insisted that recommendations for the use of screening mammography be based on statistically significant mortality reductions, alone.

The ACS group noted:

• Clinical trials have revealed mortality reductions among women ages 40-49 who have been screened with mammography.

• The Canadian trials, which showed an increase in mortality, suffered from design flaws.

• Even with Canadian trials included, a meta-analysis of all clinical trials to date demonstrates mortality reduction among women ages 40-49.

• The BCDDP, a demonstration project, showed similar survival rates for women ages 40-49, 50-59, and 60-69.

• As screening mammography has become more widespread, breast cancer in women ages 40-49 has been found at earlier stages, for which prognosis is excellent.

The NCI group noted:

• "To date, randomized clinical trials have not shown a statistically significant reduction in mortality for women under the age of 50."

Rules of Evidence

"Rules of evidence" are at the heart of the controversy. The ACS has taken a traditional course in evaluating the effectiveness of screening mammography by considering all available evidence. The strength of this position is that much evidence is available. Furthermore, as the evidence comes from independent sources, its overall consistency may be weighed to good effect. Many medical technologies in use today have been evaluated in similar fashion. The efficacy of the Pap test, for example, was determined without benefit of clinical trials.

The NCI, on the other hand, has insisted upon one standard for evaluating screening mammography: mortality reduction among women enrolled in clinical trials. The main weakness of this position is that adequate evidence does not exist from clinical trials to evaluate the effectiveness of screening mammography among women ages 40-49; it will take years to accumulate. Another weakness of the NCI position is that it was not taken a priori. Even if it makes considerable sense to adopt the clinical trials standard before a technology is widely implemented, does it still make sense to do so after considerable evidence has accumulated that the technology is in fact effective?

Sic Semper

Unfortunately, this is not merely a philosophical controversy, and at present, there are no simple answers. Breast cancer is a deadly disease, and the decision to use screening mammography for women ages 40-49 rests squarely on the primary care physician. As always, physicians must serve their patients in the face of uncertainty.

For the Record

The Rhode Island Department of Health adheres to current ACS guidelines for breast cancer screening in its Women's Cancer Screening Program.

References

1. Metlin C, Smart CR: Breast cancer detection guidelines for women ages 40 to 49 years: rationale for the American Cancer Society reaffirmation of recommendations. CA - Cancer J Clin 1994;44(4):248-255.

2. Volkers N: NCI replaces guidelines with statement of evidence. J Natl Cancer Inst 1994;86:14-15.