Prospective evaluation of periodic breast examination programs: interval cases.

A mathematical model is developed and used to obtain estimates of the expected proportion of breast cancer cases which will be detected at scheduled examinations and between them in various periodic examination programs; the latter are called interval cases. Another model is used to get estimates of the proportion of women in such programs who get cancer and are expected to have axillary node metastases at the time of first treatment. Programs with or without patient self-examinations are also evaluated. The purpose of both models is to enable evaluations of different periodic schedules. Although interval cases have been reported to have poorer prognostic characteristics than those found at scheduled examinations, calculations indicate that large reductions in these cases do no imply an equivalent reduction in positive axillary node cases. Thus, interval case counts are a relatively weak measure of the success of a periodic program. However, they may still be valuable since programs which can be expected to have low porportions of interval cases are less dependent on efficient patient self-examinations. Cost-benefit calculations show how the relative costs of mammographic and physical examinations can be considered in the development of mass screening programs. For example, if a mammographic examinations cost three times as much as a physical, then a physical examination program may be preferred to one involving both modalities.