DCIS: Ductal carcinoma in situ. A precancerous condition characterized by the clonal proliferation of malignant-looking cells in the lining of a breast duct without evidence of spread outside the duct to other tissues in the breast or outside the breast. DCIS is clearly the precursor (forerunner) of invasive breast cancer. This is evident from the sharing of clonal chromosome changes by DCIS and adjacent invasive cancers. In other words, invasive breast cancer evolves from DCIS. Also called intraductal carcinoma.

The incidence of DCIS has increased greatly, thanks to the widespread use of screening mammography. DCIS now accounts for nearly 20% of all breast cancers detected on screening mammograms. Risk factors for DCIS include older age, benign breast disease, an older age at the time of the first term pregnancy or nulliparity (not having children), and a family history of breast cancer. One in 20 women with DCIS has a mutation in the BRCA1 or BRCA2 hereditary breast cancer genes.

The crucial decision for the pathologist is to distinguish DCIS from invasive cancer. DCIS originates in a single glandular structure but may spread within the breast through the ductal system. Many patients with DCIS have discontinuous disease with gaps between the tumor foci whereas high-grade DCIS tends to be continuous.

The goal in treating DCIS is to prevent local recurrence and, in particular, invasive breast cancer. The options for surgical treatment include simple mastectomy or breast-conserving surgery (lumpectomy). Most women now have breast-conserving surgery in keeping with the detection of smaller DCIS lesions on mammography. However, after breast-conserving surgery, there is a risk for recurrence of DCIS within the breast, arising at or near the original site of the tumor.

The drug tamoxifen may be taken after surgery and radiation treatment of DCIS. Tamoxifen, a selective estrogen-receptor modulator, reduces the risk of recurrence. There is no role for chemotherapy in the treatment of DCIS.

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