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The overall incidence of DCIS in the general population is unclear. In an attempt to address this question, a small number of autopsy studies have been reported. One series examined 185 randomly selected breasts from 101 women in which a subgross sampling technique was used (6) and one or more foci of DCIS were found in 6% of cases. A review of seven autopsy series of women not known to have breast cancer during life showed a median prevalence of DCIS of 8.9% (range, 0% to 14.7%) (132). The fact that some autopsy series document a greater incidence of DCIS in asymptomatic women than most clinical series suggests either the possibility that DCIS is either underdiagnosed or that many cases are not clinically significant.
A primary consideration in the natural history of DCIS is the risk of progression to invasive carcinoma. The published evidence on the clinical course of untreated DCIS is sparse because it has been recognized as a distinct entity for only a relatively brief period, having been considered rare before the widespread use of mammography and having been treated most frequently by mastectomy. Those cases for which long-term follow-up data are available were grossly palpable DCIS, a form that may not be equivalent to the mammographic DCIS that is seen more commonly today. The few published long-term follow-up studies of DCIS after only biopsy document an overall incidence of subsequent invasive carcinoma of more than 36% (13,25,80,101). Most of these subsequent malignancies occur within 10 years, although as many as one-third may develop after 15 years (13,101).

Women with DCIS in one breast are at risk for a second tumor (either invasive or in situ) in the contralateral breast (56); the rate at which such tumors develop is similar to that among women with primary invasive breast cancer, approximately 0.5% to 1% per year.

DCIS is a part of the breast/ovarian cancer syndromes defined by BRCA1 and BRCA2, with mutation rates similar to those found for invasive breast cancer (23). These findings suggest that patients with DCIS with an appropriate personal or family history of breast and/or ovarian cancer should be screened and followed according to the same high-risk protocols as developed for invasive breast cancer.