Pathology of the Head and Neck

Major and Minor Salivary Glands Chapter 5 141Table 5.2. Salivary tumours with myoepithelial cell participation. Adapted from the WHO classification [171]BenignMalignantPleomorphic adenomaMyoepitheliomaBasal cell adenoma (some)Adenoid cystic carcinomaPolymorphous low-grade adenocarcinomaEpithelial-myoepithelial carcinomaMalignant myoepithelioma (myoepithelial carcinoma)Carcinoma ex pleomorphic adenoma (some)ally 10–50 mm in diameter, in either major or minorsalivary glands. Microscopically, there are several typicalappearances, reflecting the different forms thatneoplastic myoepithelial cells can take. Solid, myxoidand reticular growth patterns may be seen, and thecomponent cells may be spindle-shaped, plasmacytoid(hyaline), clear, epithelioid or oncocytic. Manytumours show more than one growth pattern or celltype, but myoepitheliomas of the minor glands aremore often composed of plasmacytoid cells, and thoseof the parotid spindle cells [189]. Although most authorsaccept the plasmacytoid cells as myoepithelial,it has recently been suggested that these cells originatefrom luminal and not from myoepithelial cells [157],and thus the tumours should possibly be reclassifiedas plasmacytoid adenomas [157]. The clear cell variantcan occur in both major and minor glands [182], butis relatively rare [43]. Unlike their malignant counterpart[52] (see Sect. 5.9.8), benign myoepitheliomas donot usually show invasiveness, necrosis, cytologicalpleomorphism, or more than an isolated mitotic figure.The stroma is usually scanty, fibrous or myxoid,and it may occasionally contain chondroid material ormature fat cells [203]. Extracellular collagenous crystalloidsare seen in 10–20% of plasmacytoid cell-typemyoepitheliomas, (as well as sometimes in myoepithelial-richPAs); these structures are about 50–100 min diameter and consist of radially-arranged needleshapedfibres composed of collagen types I and III,which stain red with the van Gieson method [197].Scanty small ducts may be present (usually less than10% of the tumour tissue) in otherwise typical myoepitheliomas(Fig. 5.14) [43]. Immunohistochemically,almost all tumours express S-100 protein, as wellas some cytokeratins, especially subtype 14. Alphasmooth muscle actin positivity is seen to some degreein most spindle cell myoepitheliomas, but only occasionallyin the plasmacytoid cell type [189]. Stainingfor calponin, smooth muscle myosin heavy chain(SMMHC) and CD10 is inconsistent in myoepithelialcells. The nuclear transcription factor p63 is positivein most benign myoepitheliomas [166]. Electron microscopicstudies have also confirmed both epithelialFig. 5.15. Basal cell adenoma. The tumour is arranged in nests, islandsand trabeculae or basal cells without cytological abnormality.Ductal differentiation is also notedand smooth muscle differentiation [170], although focaldensities in myofilaments are not usually found[43]. The behaviour of myoepithelioma is similar tothat of pleomorphic adenoma, and complete excisionshould be curative. Neither growth pattern nor celltype appears to carry prognostic significance. Malignantchange in a benign lesion has been described [2],but too little information is available about the percentageof cases involved. However, it is reasonable topostulate that it is probably not very different fromthat of pleomorphic adenoma.5.8.3 Basal Cell AdenomaICD-O:8147/0Most tumours previously described as monomorphicadenoma are now termed basal cell adenoma (BCA).The revised WHO [171] classification recognises fourhistopathological subtypes – solid, tubular, trabecularand membranous – but it is likely that, in reality, thereare only two separate biological entities [16] – membranousand non-membranous (Figs 5.15, 5.16).