Breast cancer screening by mammography: an overview

Given the differences between the nine studies in design and execution alone, it is remarkable that they all support the same general conclusion that mammographic screening for breast cancer is capable of reducing mortality from the disease. There are also differences between them in the screening technique (age range, interval between screens, number of mammographic views) and treatment but this does not provide a simple guide to what we can expect in the National Health Service Breast Cancer Screening Programme which is now underway in this country. The new national programme recruits women in the age range 50-64 and Table 3 shows the result from five of the studies in which separate data for a 'middle-age' group can be derived: the total Utrecht study group was aged 50-69 and the other three cannot be divided in this way. In each study, apart from the HIP study, the magnitude of the benefit is greater than in the total group. It is possible that these differences will decrease with longer follow-up (it took longer for an effect to emerge in the youngest age group in the HIP study) but the relative risks in Table 3 give a better guide than those in Table 2 in the short term. The specific effects of different intervals between screens and mammographic views are currently being investigated in randomized clinical trials. In the meantime our best estimate of what is likely to happen in the national programme is probably provided by the two-counties study which, like us, used single-view mammography and in the 50-64 age group had an average screening interval of 33 months-close to the 3 years we have adopted.