Effect of different mammographic radiation exposures on predicted benefits of screening for breast cancer.

The benefits and risks of screening for breast cancer, including both mammographic and clinical examinations, are analysed by means of a probabilistic model. The model, applied to previously unpublished data provided by the HIP (Health Insurance Plan of Greater New York) breast cancer screening project, permits more precise stratification by age than previous analyses and allows for incorporation of oncogenic risks due to mammography. It is assumed that, from the tenth year of screening onward, radiation induces 6 cases of breast cancer per year per million women screened per rad delivered to the breast tissue per exam. The predicted pattern of breast cancer cases and deaths is commensurate with that actually observed in the first nine years of the HIP study. In extrapolation to lifetime experience, the model predicts an ultimate decrease in the probability of dying of breast cancer only for women over age fifty at initial screening and an increase in life expectancy for the entire screened group, at an exposure level of three rads per exam. Had radiation dosage been one rad or less per exam, which is within the range now possible with modern equipment and procedures, the radiation risk would not offset even a modest benefit such as is predicted for the younger women, either in number of deaths or years of life expectancy.