MAMMOGRAPHIC DENSITIES AS A CRITERION FOR ENTRY TO A CLINICAL-TRIAL OF BREAST-CANCER PREVENTION

The most convincing evidence that a factor such as dietary fat is caus ally related to breast cancer would be obtained from a randomised cont rolled trial in which exposure to dietary fat intake was systematicall y varied. A limitation of randomised controlled trials of breast cance r prevention, however, is the large sample size required to detect pla usible reductions in risk resulting from the intervention. We describe here experience over a period of 9 years with the use of one risk fac tor for breast cancer as a criterion for entry to a clinical trial of breast cancer prevention. The risk factor used was the presence of ext ensive densities in the breast tissue on mammography, which has been f ound by several investigators to be strongly associated with risk of b reast cancer. Using this criterion for selection, 1800 subjects of mea n age 46 years were enrolled between 1982 and 1986, and again between 1988 and the present. Throughout this period, the point estimate of an nual invasive cancer incidence was approximately 6 per 1000 per year. The observed cancer incidence has been consistently 4-5 times the inci dence expected from age-specific breast cancer incidence data for wome n living in Ontario. These data show that the selection of subjects fo r a clinical trial of breast cancer prevention using the criterion of extensive breast parenchymal densities does identify a group at substa ntially increased risk of breast cancer. Use of this criterion for the selection of subjects can substantially reduce the sample size requir ed for a clinical trial of a preventive strategy.