Ductal carcinoma in situ-like structures in metastatic breast carcinoma

In this report, we present two cases of axillary lymph node metastatic breast carcinoma with features mimicking ductal carcinoma in situ (DCIS): one was of the comedo-like type and the other was suggestive of the micropapillary type. In the first case, the primary tumor presented DCIS of the comedo type; however, in the second case, the primary tumor consisted only of the invasive ductal component. Immunohistochemistry against smooth muscle actin, S100-protein, CK5/6, CD10, P63, and 34βE12 did not identify myoepithelial cells either in DICS of the first primary tumor or in both metastases. These features probably do not represent the true DCISs, but only mimic them. This observation suggests that a proportion of “primary DCIS” may constitute an invasive pseudo-DCIS carcinoma, and immunohistochemical identification of myoepithelial cells may be helpful in such cases.