Pathology of the Head and Neck

140 S. Di Palma · R.H.W. Simpson · A. Skalova · I. Leivo5Fig. 5.13. Recurrent pleomorphic adenoma. Multiple and oftenwell-separated tumour nodules of different sizes are seen in theperiparotid soft tissueFig. 5.14. Benign myoepithelioma composed of plasmocytoid(hyaline) and epithelioid cells with areas of myxoid stroma. Plasmocytoidcells have eccentric nuclei and dense eosinophilic cytoplasmthese neoplasms should also be sampled thoroughly. Tumourcells in lymphatics (“vascular invasion”) are occasionallyseen in benign PAs, but this does not necessarilyindicate malignancy (Fig. 5.12) [3]. None of the reportedcases were followed by metastases.Pleomorphic adenomas are often completely or partlysurrounded by a fibrous capsule of variable thickness,but it can be absent, especially in tumours of the minorglands. Neoplastic elements may extend into and eventhrough the capsule in the form of microscopic pseudopodiaor apparent satellite nodules.They may be the cause of future recurrence after apparentsurgical removal [97], and their presence shouldbe noted in the surgical pathology report. Special stainsand immunohistochemistry are not necessary for thediagnosis in most cases, but can be used to identify thedifferent cell types and also early malignant change (seeSect. 5.9.11).Recurrent PA occurs after incomplete surgical excisionand is usually composed of multiple nodules completelyseparate from each other. In the first recurrencethe nodules are usually seen within salivary gland tissue,but in further recurrences tumours are found in thesoft tissue of the surgical bed (Fig. 5.13). Histologically,the nodules show similar features to ordinary PA, andin particular they lack any cytological atypia. In spiteof this, confluent nodules of recurrent PA can still killthe patient. As discussed later (see Sect. 5.9.11) multiplyrecurrent PAs may rarely metastasise to distant sites,and in addition are more prone to developing malignantchanges.5.8.1.1 Salivary Gland Anlage Tumour(“ Congenital PleomorphicAdenoma”)ICD-O:8940/0This is a rare, probably hamartomatous lesion in the nasopharynxof neonates [45]. Although potentially fataldue to its location, prognosis after surgery is good. It wasnot included in the 1991 WHO classification [171]. Themicroscopic features are a biphasic pattern of squamousnests and duct-like structures at the periphery, merginginto solid, predominantly mesenchymal nodules, possiblyof myoepithelial origin. Occasionally, there is necrosisand cyst formation [136].5.8.2 Benign MyoepitheliomaICD-O:8982/0Myoepithelial cells are found in several salivary glandneoplasms (Table 5.2). Benign myoepithelioma wasfirst described in 1943 [179], and was included inthe 1991 revised WHO classification [171]. It can bedefined as a tumour composed totally, or almost totally,of myoepithelial cells. Whether or not it is trulya separate biological entity is debatable, but mostcommentators believe that it represents one end of aspectrum that also includes pleomorphic and at leastsome basal cell adenomas. Nevertheless, myoepitheliomadisplays particular microscopic features thatpose specific practical problems in the identificationand differential diagnosis, and on this basis it can beaccepted as a separate diagnostic category [188, 189].Most cases present as a well-circumscribed mass, usu-