periparotid soft tissue mocytoid cells have eccentric nuclei and dense eosinophilic cytoplasm these neoplasms should also be sampled thoroughly. Tumour cells in lymphatics ("vascular invasion") are occasionally seen in benign PAs, but this does not necessarily indicate malignancy (Fig. 5.12) . None of the reported cases were followed by metastases.
Pleomorphic adenomas are often completely or partly surrounded by a fibrous capsule of variable thickness, but it can be absent, especially in tumours of the minor glands. Neoplastic elements may extend into and even through the capsule in the form of microscopic pseudo-podia or apparent satellite nodules.
They may be the cause of future recurrence after apparent surgical removal , and their presence should be noted in the surgical pathology report. Special stains and immunohisto chemistry are not necessary for the diagnosis in most cases, but can be used to identify the different cell types and also early malignant change (see Sect. 5.9.11).
Recurrent PA occurs after incomplete surgical excision and is usually composed of multiple nodules completely separate from each other. In the first recurrence the nodules are usually seen within salivary gland tissue, but in further recurrences tumours are found in the soft tissue of the surgical bed (Fig. 5.13). Histologically, the nodules show similar features to ordinary PA, and in particular they lack any cytological atypia. In spite of this, confluent nodules of recurrent PA can still kill the patient. As discussed later (see Sect. 5.9.11) multiply recurrent PAs may rarely metastasise to distant sites, and in addition are more prone to developing malignant changes.
184.108.40.206 Salivary Gland Anlage Tumour ("Congenital Pleomorphic Adenoma")
This is a rare, probably hamartomatous lesion in the nasopharynx of neonates . Although potentially fatal due to its location, prognosis after surgery is good. It was not included in the 1991 WHO classification . The microscopic features are a biphasic pattern of squamous nests and duct-like structures at the periphery, merging into solid, predominantly mesenchymal nodules, possibly of myoepithelial origin. Occasionally, there is necrosis and cyst formation .
5.8.2 Benign Myoepithelioma
Myoepithelial cells are found in several salivary gland neoplasms (Table 5.2). Benign myoepithelioma was first described in 1943 , and was included in the 1991 revised WHO classification . It can be defined as a tumour composed totally, or almost totally, of myoepithelial cells. Whether or not it is truly a separate biological entity is debatable, but most commentators believe that it represents one end of a spectrum that also includes pleomorphic and at least some basal cell adenomas. Nevertheless, myoepithe-lioma displays particular microscopic features that pose specific practical problems in the identification and differential diagnosis, and on this basis it can be accepted as a separate diagnostic category [188, 189]. Most cases present as a well-circumscribed mass, usu-
Table 5.2. Salivary tumours with myoepithelial cell participation. Adapted from the WHO classification 
Benign Pleomorphic adenoma
Myoepithelioma Basal cell adenoma (some)
Polymorphous low-grade adenocarcinoma Epithelial-myoepithelial carcinoma Malignant myoepithelioma (myoepithelial carcinoma) Carcinoma ex pleomorphic adenoma (some)
ally 10-50 mm in diameter, in either major or minor salivary glands. Microscopically, there are several typical appearances, reflecting the different forms that neoplastic myoepithelial cells can take. Solid, myxoid and reticular growth patterns may be seen, and the component cells may be spindle-shaped, plasmacy-toid (hyaline), clear, epithelioid or oncocytic. Many tumours show more than one growth pattern or cell type, but myoepitheliomas of the minor glands are more often composed of plasmacytoid cells, and those of the parotid spindle cells . Although most authors accept the plasmacytoid cells as myoepithelial, it has recently been suggested that these cells originate from luminal and not from myoepithelial cells , and thus the tumours should possibly be reclassified as plasmacytoid adenomas . The clear cell variant can occur in both major and minor glands , but is relatively rare . Unlike their malignant counterpart  (see Sect. 5.9.8), benign myoepitheliomas do not usually show invasiveness, necrosis, cytological pleomorphism, or more than an isolated mitotic figure. The stroma is usually scanty, fibrous or myxoid, and it may occasionally contain chondroid material or mature fat cells . Extracellular collagenous crystalloids are seen in 10-20% of plasmacytoid cell-type myoepitheliomas, (as well as sometimes in myoepithe-lial-rich PAs); these structures are about 50-100 p,m in diameter and consist of radially-arranged needle-shaped fibres composed of collagen types I and III, which stain red with the van Gieson method . Scanty small ducts may be present (usually less than 10% of the tumour tissue) in otherwise typical myo-epitheliomas (Fig. 5.14) . Immunohistochemical-ly, almost all tumours express S-100 protein, as well as some cytokeratins, especially subtype 14. Alpha smooth muscle actin positivity is seen to some degree in most spindle cell myoepitheliomas, but only occasionally in the plasmacytoid cell type . Staining for calponin, smooth muscle myosin heavy chain (SMMHC) and CD10 is inconsistent in myoepithelial cells. The nuclear transcription factor p63 is positive in most benign myoepitheliomas . Electron microscopic studies have also confirmed both epithelial
and smooth muscle differentiation , although focal densities in myofilaments are not usually found . The behaviour of myoepithelioma is similar to that of pleomorphic adenoma, and complete excision should be curative. Neither growth pattern nor cell type appears to carry prognostic significance. Malignant change in a benign lesion has been described , but too little information is available about the percentage of cases involved. However, it is reasonable to postulate that it is probably not very different from that of pleomorphic adenoma.
S.B.3 Basal Cell Adenoma
Most tumours previously described as monomorphic adenoma are now termed basal cell adenoma (BCA). The revised WHO  classification recognises four histopathological subtypes - solid, tubular, trabecular and membranous - but it is likely that, in reality, there are only two separate biological entities  - membranous and non-membranous (Figs 5.15, 5.16).
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