A cytologic, immunocytochemical and ultrastructural study of a malignant parotid gland myoepithelioma

A 38-year-old man presented with a mass in the superficial lobe of the left parotid gland. One year before he underwent a total thyroidectomy with cervical node dissection for a papillary carcinoma, pT2 N0. The fine needle aspiration (FNA) cytology of the parotid mass showed large sheets of cells, with irregular borders and nuclear overlapping, and single malignant cells, in a bloody background devoid of metachromatic substance. Two cell types were evident: epithelioid cells, mostly single, characterized by an eccentric, occasionally kidney-shaped or bilobated nucleus, with round or polygonal, abundant and homogeneous cytoplasm, basophilic with the May–Grünwald–Giemsa (MGG) stain (Figure 1a); and spindle-shaped cells, with bipolar form, aggregated in cohesive sheets, showing a centrally placed oval vesicular nucleus, with elongated fibrillary cytoplasm (Figure 1b). The nuclear texture was coarsely granular with a prominent nucleolus or multiple nucleoli and an irregular nuclear membrane. Scattered atypical mitotic figures were present. Occasionally normal serous acini were intermingled within the neoplastic proliferation. A diagnosis of malignant cells, not otherwise specified (NOS) was made. A superficial left parotidectomy was performed. The lobe contained a reddish, translucent, omogeneous mass, 3.5 cm in diameter, with ill defined borders (Figure 2). Histological examination of the specimen confirmed the absence of any capsule: the neoplastic proliferation infiltrated the fibrofatty tissue, the salivary gland (Figure 3) and the perineural spaces. The neoplastic cells, with a predominantly spindle shape, were haphazardly orientated with irregularly interwoven fascicles (Figure 4), delimiting nests of discohesive epithelioid cells, merging with a hyalinized or myxoid stroma (Figure 5a). The elongated processes of the spindle-shaped cells blended with hyalinized, fibrillary extracellular matrix (Figure 5b). The tumoural mass grew in a wide reticulin network. Both