Histopathology

EMC has a lobulated growth pattern with a mixed tubular and solid architectural arrangement. Papillary and cystic areas can be identified in around 20% of the cases. Tumours from minor salivary and sero-mucinous glands show infiltration of surrounding tissues and there is ulceration of the overlying mucosa in about 40% of the cases.

The hallmark of EMC histology is the presence of bi-layered duct-like structures: the inner layer is formed by a single row of cuboidal cells, with dense, finely granular cytoplasm and central or basal, round nucleus. The outer layer may show single or multiple layers of polygonal cells, with well-defined borders; the cytoplasm is characteristically clear and the nucleus is vesicular and slightly eccentric. The double-layered pattern is preserved in papillary-cystic areas but solid tumour areas may be exclusively formed by clear cells. PAS positive, hyaline, eosinophilic strands of basement membrane-like material surround the duct-like structures and, in solid areas, divide the clear cells into the-ques. Coagulative necrosis at the centre of tumour nodules is uncommon. In rare cases, squamous differentiation and spindle cells are observed as well as an oncocytic appearance in the inner cell layer of neoplastic ducts. Perineural and vascular invasion are frequent and bone invasion may occur. None to 1-2 mitoses per 10 HPF can be identified in the clear cell population of EMC. Rare cases of dedifferentiation have been reported {42,783}.

Immunoprofile

Myoepithelial markers (smooth muscle actin, HHF35, p63 and/or calponin) stain the clear cell compartment. The luminal cells stain with cytokeratins.

Fig. 5.10 Epithelial-myoepithelial carcinoma (EMC) of the parotid gland. A Double layered architecture formed by an inner layer of eosinophilic cuboidal cells and an outer layer of clear, myoepithelial-type cells. B Dedifferentiated EMC. There is co-existence of areas typical of EMC (left) with areas of undifferentiated carcinoma (right).

Fig. 5.10 Epithelial-myoepithelial carcinoma (EMC) of the parotid gland. A Double layered architecture formed by an inner layer of eosinophilic cuboidal cells and an outer layer of clear, myoepithelial-type cells. B Dedifferentiated EMC. There is co-existence of areas typical of EMC (left) with areas of undifferentiated carcinoma (right).

Epithelial-myoephithelial carcinoma 225

Fig. 5.11 Epithelial-myoepithelial carcinoma. A Ductal tumour cells of inner layer predominance. B Solid growth of clear, myoepithelial-type cells.

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