The canadian national breast screening study: Responses to controversy

Abstract

A t least since 1985, there has been concern in some quarters that the Canadian National Breast Screening Study (NBSS) results 1'2 would fail to show a benefit from combined mammographic and clinical screening for breast cancer in women age 40-49 years. In the United States, in the spring of 1992, the controversy about the benefits of screening women age 40-49 for the early detection of breast cancer erupted in the media. 3-7 The center of attention was the unpublished results of the NBSS. However, the center of controversy is the conflicting guidelines on breast screening for women age 40-49. The very familiar guidelines of the American Cancer Society and the American College of Radiology (ACR) exhort women in this age group to do themselves a favor and get a mammogram. Although a number of prestigious professional associations have chosen to endorse these guidelines, others have not, including the US Preventive Services Task Force, the American College of Physicians, the Canadian Workshop Group, 8 the National Cancer Institute of Canada, an expert panel in the United Kingdom, 9 and the European Group for Breast Screening. 1° To understand the nature of the controversy, it is helpful to review both the NBSS and published evidence from other screening studies. The Canadian NBSS was designed to answer two major questionsJ ,2 First, in women age 40-49, does annual mammography combined with annual physical examination of the breast for 3 or 4 years reduce the mortality rate from breast cancer as compared with a single physical examination alone? Second, in women age 50-59, how much does mammography contribute to mortality reduction over and above that achieved by physical examination alone when the comparison is annual mammography combined with physical examination versus annual physical examination alone? The NBSS screened 89,835 women at 15 centers across Canada from 1980-1988, with each woman individually randomized to the mammography arm or the nonmammography arm of the study. Approximately 50,000 participants were 40-49 on entry and the remainder were 50-59 years of age. To be eligible, women were required to sign informed consent forms, to never have had a diagnosis of breast cancer, © 1992 by The Jacobs Institute of Women ' s Health 1049-3867/92/$5.00