Lymphoepithelioma-like carcinoma of the breast. Report of a case with the first electron microscopic study and review of the literature.

Lymphoepithelioma-like carcinoma (LELC) is a tumor which occurs outside the nasopharynx and has morphological features identical to nasopharyngeal lymphoepithelioma. LELC of the breast (LELC-B) is uncommon, and its resemblance to medullary carcinoma of the breast (MC-B) obscures distinction between these two tumors. We report a case of LELC-B occurring in a 47-year-old woman. The tumor consisted of multinodules without circumscription. The tumor cells mainly exhibited loose clusters being permeated by numerous lymphocytes. The tumor cell clusters showed inconspicuous margins, which were far from syncytial patterns. The epithelial nature of the tumor cells was demonstrated by positivity for epithelial membrane antigen, AE1/AE3 and CAM5.2. Furthermore, glandular differentiation of the tumor cells was confirmed using electron microscopy for the first time. Epstein-Barr virus (EBV) was not detected using either in situ hybridization or polymerase chain reaction. These findings, together with former reports of LELC-B, suggest that the distinction between LELC-B and MC-B depends on whether circumscription and syncytial growth patterns exist. The other findings, including absence of EBV and immunohistochemical aspects of the tumor cells, are not considered different thus far. Although the prognosis of LELC-B is thought to be favorable, which is also similar to MC-B, distant metastasis was detected in the present case. To confirm the clinicopathological entity of these two tumors, it is important to recognize the difference between LELC-B and MC-B.