Fig. 2

microscopic features of MEC-b [hematoxylin and eosin (H&E) and immunohistochemistry]. A The tumor cells were mostly arranged in solid nests admixed with necrotic areas (upper left and right corner). The tumor cell population was characterized by a mixture of large epidermoid cells and relatively small intermediate cells in absence of mature keratinization. B Microcystic and cribriform structures lined by tumor cells with mucinous differentiation, floating in large pools of extracellular mucine. C At high power magnification the heterogeneous tumor cell composition was clearly visible. Next to large epidermoid cells and intermediate cells, we noticed also the presence of scattered cells with clear cytoplasm and mucinous differentiation. The adjacent stroma showed moderate mixed inflammatory infiltrate characterized by high number of neutrophils. The formation of several micro-abscesses was also apparent. The typical zoning pattern described in MEC was clearly visible by sequential staining with CK 5.6 (D) and CK7 (E). The two microphotographies show a mirror picture with large epidermoid cells positive for CK5.6 but negative for CK7, and conversely the mucinous component positive for CK7 but negative for CK 5.6. F The HER2 immunostaining surprisingly showed strong and diffuse membranous staining in all tumor cells. Fluorescence in situ hybridization (FISH) analysis confirmed the amplification of the HER2 gene