DCIS now represents about 14 percent, or 1 in 7, of all new breast cancer diagnoses in the United States.96,114 Among screen-detected breast cancers, 20 percent are DCIS and 1 in every 1,300 screening mammograms leads to a diagnosis of DCIS.22,d When younger women are diagnosed with breast cancer, it is somewhat less likely to be DCIS (Table 2-2). All breast cancers are more common among older women, so relatively and absolutely more of DCIS is found in older women.
Before the widespread use of screening mammography, many cases of DCIS went unrecognized; the increased numbers and proportion of DCIS cases that are now recognized do not necessarily mean that more women are developing DCIS. The reported incidence of DCIS is determined by the actual number of cases and also by the ability to detect them.
DCIS is not life-threatening per se, but it is a significant risk factor for invasive breast cancer. It is believed to precede the development, over time, of invasive breast cancer,54 although the rate of development can be so slow that it never becomes life threatening.46,77 Among women in an extensive mammography registry study who were initially diagnosed with DCIS between 1978 and 1983, 3.4 percent died of invasive breast cancer within 10 years; of those diagnosed between 1984 and 1989, the 10-year breast cancer mortality was 1.9 percent.22 These rates, which are approximately one-tenth those for women diagnosed with localized invasive breast cancer, may reflect the effectiveness of treatment for DCIS, the mildness of the condition, or both. Deaths from breast cancer among women with DCIS are thought to result from an invasive component that was not recognized at the time of the DCIS diagnosis or because of progression to an invasive cancer.
The most important issue for DCIS is not, however, the increased detection, but rather the information, which mammography cannot provide, that would permit optimally individualized treatments. Cessation of screening mammography, or any other screening modality, would not solve the problem of overtreatment.26 Instead, the solution lies in tailoring treatment to the biological characteristics of individual cases.
DCIS occurs when malignant epithelial cells proliferate within the breast ducts but remain confined by the basement membrane (a thin non-