Myoepithelial carcinoma (MC) consists almost entirely of modified myoepithelial cells. A malignant salivary gland tumour consisting of myoepithelial cells but containing more than the occasional true luminal cells should not be classified as an MC. This definition is in line with that separating its benign counterpart, the myoepithelioma, from pleomorphic adenoma. MC is distinguished from myoepithelioma by its infiltrative, destructive growth. MC is a rare tumour, and in 1996 Ellis and Auclair found only 35 cases from the English literature and the AFIP register. Furthermore, they note that MC only constituted 0.2 % of all salivary gland neoplasms in their register during a 10-year period [14]. Although MC is a quite rare tumour, clinicopathological studies during the last 15 years indicate that it may not be as extremely rare as considered previously. The largest series before 1996 was that reported in 1993 by Di Palma and Guzzo and that consisted of ten cases [10]. In their review of the literature, Di Palma and Guzzo found less than 100 reported cases of myoepithelial tumours, the vast majority of which were benign or locally aggressive and very few had overt malignant features. In fact, less than 20 cases were reported as MC before 1993 [8, 10, 11, 24, 28, 56, 60, 61, 63]. This paucity is partly due to the fact that in the older literature many cases of MC were likely reported as carcinoma ex pleomorphic adenoma rather than subclassified as myoepithelial carcinoma. MC is nevertheless a rare salivary malignancy and is today considered to represent less than 2 % of salivary gland carcinomas [57].

MC is predominantly a parotid tumour. The majority of cases, 60–70 %, have arisen in the parotid gland. The minor salivary glands are the primary site in 25 % of cases, whilst approximately 10 % arise in the submandibular gland [9, 10, 14, 26, 39, 47, 68]. MC arises de novo or from a pre-existing benign tumour, most commonly from pleomorphic adenoma but also from myoepithelioma. In 14 of the 25 cases of MC reported by Savera and associates [47], there was unequivocal histological evidence of underlying benign tumour. Twelve of these were pleomorphic adenoma and two were myoepithelioma. Similarly, Di Palma and Guzzo [10] reported five of their ten cases to have arisen in a pre-existing pleomorphic adenoma and five de novo. The study by Alós and associates [1] comprised 16 myoepithelial neoplasms, four of which were MC and all of which had developed from pre-existing pleomorphic adenoma and myoepithelioma. From the literature it hence appears that MC arises more frequently from a pre-existing benign salivary gland tumour than de novo. As for most intraoral salivary gland tumours, its benign counterpart myoepithelioma inclusive, the palate is the most common intraoral site for MC.

Myoepithelial tumours arising in minor salivary glands outside the oral cavity is an exceedingly rare event, but MC has been reported in the larynx [20, 28], nasopharynx [41, 64], lungs [35] and even a rare case in the infratemporal fossa [21]. Myoepithelial carcinoma may also arise in the skin and soft tissue [3, 25, 36, 59].

Due to its rarity and relatively small series studied, little information on the clinical signs and symptoms of MC has been obtained. The largest study so far comprised 25 patients, and in most of those cases, the tumour presented as a painless mass [47]. MC may be locally destructive and the tumour size varies usually being between 2 and 10 cm, although huge tumours up to 20 cm have been described. MCs are devoid of a true capsulate but can be well circumscribed and usually have a grey-whitish cut surface, occasionally with necrosis and cystic degeneration. In contrast to the slight female predilection observed for myoepithelioma, males and females appear to be affected equally. The mean age of presentation is the sixth decade but shows a very wide age distribution (range 14–86 years) [1, 10, 14, 47].

Epithelial-myoepithelial carcinoma (EMC) is a tumour consisting of various proportions of two cell types which typically form duct-like structures. There is an inner layer of duct-lining cells and an outer layer of clear myoepithelial cells [52]. The clear cells are often relatively large and usually predominate. The term epithelial-myoepithelial carcinoma was introduced in 1972 by Donath and associates [13] and later firmly established after the report of 16 cases in 1982 by Corio and associates [7]. EMC was recognised as a separate entity in the 2nd WHO classification in 1991 [52] but had before that been described under various names, e.g. glycogen-rich adenoma [23], glycogen-rich adenocarcinoma [38], clear cell adenoma [45] and clear cell carcinoma [5].

EMC is like myoepithelial carcinoma, a rare salivary malignancy and also constitutes 1–2 % of salivary carcinomas [15, 17]. Most publications are case reports [27, 29, 31, 42, 62, 66], but also a smaller number of clinicopathological studies have been published during the last two decades [6, 40, 43, 53]. The clinicopathological features and immunophenotypic characteristics of 61 cases of EMC were reported by Seethala and associates [49], which is the largest series of epithelial-myoepithelial carcinomas published so far.

Typically, the EMC is a low-grade malignancy and occurs mainly in the parotid gland [17], although it is described in mucoserous glands of the tongue [31], buccal mucosa [65], sinonasal tract [49] and low aerodigestive tract as well [12, 19, 40, 44].

Grossly, myoepithelial carcinomas (MC) are unencapsulated but may be well defined with nodular surfaces. The cut surface is grey-white and can be glassy. Some tumours reveal areas of haemorrhage, necrosis and pseudocystic degeneration.

Histologically, MCs are characterised by multilobulated growth pattern, not infrequently with necrotic foci, with infiltration into adjacent tissues (Fig. 13.1). Perineural and vascular invasion is seen in 44 and 16 %, respectively. The tumour-related extracellular matrix is generally prominent and is myxoid or hyalinised. There is a considerable range of microscopic features both in architecture and cytology, reflecting the different forms of modified myoepithelial cells. The nuclei of MC may be relatively uniform, small to intermediate sized and composed of finely distributed chromatin, lacking obvious nucleoli, or there may be marked cytological atypia, with enlarged pleomorphic nuclei, showing chromatin clumping and large nucleoli. Multinucleated and bizarre tumour giant cells may occasionally be present.

The neoplastic nodules comprise solid, reticular and sheet-like growth patterns, often with plentiful myxoid (Fig. 13.2) and hyaline extracellular matrix (Fig. 13.3). The range of cell types includes epithelioid cells often arranged in trabecular and pseudoacinar and duct-like cleft formations within abundant myxoid stroma (Fig. 13.4), cells with clear cytoplasm sometimes lipoblast-like, and sometimes appearing signet ring-like, hyaline-plasmacytoid and spindle-shaped cells (Fig. 13.5). In most MCs, one cell type predominates, but there is usually a minority of other cell types, too. Metaplastic changes are frequently seen and include squamous, chondroid and sebaceous features. Mitotic figures may be scanty to plentiful and include atypical forms. A unique case of MC of minor salivary gland with numerous collagenous crystalloids has been reported [4].

Grossly, the majority of tumours display a multinodular growth pattern, pushing borders, and are partially to completely encapsulated. The infiltrative growth pattern of EMC is less common, observed in about 12 % [49].