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the history will usually be that of rapid growth in a longstanding salivary nodule.

Microscopy shows a biphasic tumour composed of epithelial and mesenchymal elements. The former is generally a poorly differentiated (adeno)carcinoma, but salivary duct carcinoma is increasingly reported (Fig. 5.52) [49, 50]. The other component is usually a chondrosarcoma, but osteogenic sarcoma, fibro-sarcoma, malignant fibrous histiocytoma, pleomor-phic rhabdomyosarcoma and osteoclast-type giant cell neoplasms [214, 224] have also been described.

Epithelial markers are usually detected in the epithelial component, which may or may not also be expressed in the sarcomatous component. Positive staining for epithelial markers has been used as proof of the fact that CS are carcinomas showing divergent differentiation and as an indication of their monoclonal origin. However, keratin staining can be negative casting doubt onto the monoclonal-carcinomatous nature of the whole tumour. Molecular studies have been helpful in clarifying this issue. In analogous neoplasms in other organs such as breast, uterus and in salivary glands, molecular studies have demonstrated that car-cinomatous and sarcomatous components have similar genetic profiles. Moreover, in a subset of CS with osteoclastic-type giant cells, Tse et al. [224] found mutation of the same allele on chromosome 17p13, which is a known mutation of salivary duct carcinoma. This indicates that carcinosarcoma are in fact carcinomas of high-grade malignancy and should be treated as such. HER-2 overexpression on immunohistochemis-try is also seen in the salivary duct component of CS. The meaning of this information has still not been clarified [57].

metastases [232]. Although the morphology of both is almost identical, the recurrences seem to play an important role in the genesis of systemic spread. This suggests that surgical manipulation may favour vascular implantation or invasion eventually leading to metastases, but in many cases of MPA it was not possible his-tologically to demonstrate actual vascular permeation [56, 232].

5.9.12 Sebaceous Carcinoma

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Although sebaceous glands are common in the oral mucosa (Fordyce granules), sebaceous neoplasms of the salivary glands are rare. Most sebaceous carcinomas have arisen in the parotid [62], possibly from pluripotent duct cells [219]. The sex incidence is equal, and the mean age is 69 years (range 17-93). Macroscopically, they are partly encapsulated and vary in size from 6 to 85 mm across the greatest diameter. Microscopy shows invasive islands, duct-like structures and sheets of tumour cells, which may be sebaceous, squamous or basaloid; intracellular mucin may be found [12]. Sebaceous cells are present in varying numbers, and typically comprise foamy cytoplasm and a single vesicular nucleus with a prominent nucleolus. Areas of necrosis are frequent [85]. The tumour cells react with cytokeratin and EMA, but not with S-100 protein or actin [219]. The behaviour is intermediate to high-grade, and recurrences, metastases and death due to disease have all been reported. Three cases of sebaceous lymphadenocarcinoma have been described, representing malignant transformation of sebaceous lymphadenoma. One of the patients died because of the tumour [62].

5.9.11.3 Metastasising

Pleomorphic Adenoma

5.9.13 Lymphoepithelial Carcinoma

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This tumour is histologically indistinguishable from benign PA, yet it metastasises widely to sites including lymph nodes, bone, lung and kidney, and can kill the patient [232]. Whereas the WHO revised classification lists metastasising pleomorphic adenoma (MPA) as one entity in the MMT category of [171], it differs because it remains histologically "benign" in the primary site, local recurrences, and metastatic deposits [56, 232].

It is a rare tumour with fewer than 100 reported cases so far. Despite this, MPA has a clear-cut clinicopath-ological profile: the reported cases shared several similarities, such as long time intervals (up to 50 years) between the primary tumour and metastases, and simultaneous, usually multiple, local recurrences and distant

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The WHO revised classification includes this tumour under undifferentiated carcinomas [171], but it is a genuine clinicopathological entity and can be considered separately.

Lymphoepithelial carcinoma is extremely rare except in Eskimos (Inuit) and in Southern China. The median age is 40 years (range 10-86), and it is slightly commoner in females [62]; familial clusters have been identified amongst patients from Greenland [1]. The parotid is involved in 80% of cases, with the rest occurring in the submandibular glands. Forty per cent of patients have lymph node metastases at the time of presentation. A few examples have been described in association with lymphoepithelial sialadenitis [121], but a much more important association is with Epstein-Barr

Table 5.4. Metastases to the parotid gland, adapted from Gnepp [82]

Location of primary

Skin of head and neck

Upper aero-digestive tract (mouth, nose, sinuses, pharynx)

Eye (conjunctiva, lacrimal gland)

Thyroid

Head, not otherwise specified

Central nervous system

Submandibular salivary gland

Lung

Kidney

Breast

Colorectal

Prostate

Skin, distant

Stomach

Uterus

Pancreas

Total, distant sites

Skin, not otherwise specified

Unknown primary site virus, and viral genomes can be detected in the malignant cells [89].

There is a marked histological similarity to undif-ferentiated nasopharyngeal carcinoma, which has also been linked to Epstein-Barr virus. Microscopic examination shows syncytial groups of large epithelial cells with vesicular nuclei and prominent nucleoli, intimately mixed with lymphocytes and plasma cells, sometimes with germinal centre formation. Mitotic figures are often numerous. At times, the epithelium is difficult to identify, but it can be highlighted by cy-tokeratin markers. The most important differential diagnosis is a metastasis from a nasopharyngeal primary, which can present as a parotid mass [231], or possibly very poorly differentiated squamous carcinoma of usual type originating in the skin or upper aerodiges-tive tract. The outcome is surprisingly good for such an aggressive-looking carcinoma, and the 5-year survival rate is 60%.

5.9.14 Small Cell Carcinoma

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Small cell carcinoma (SCC) is unlikely to be a single entity, as electron microscopy reveals that some neoplasms show neuroendocrine differentiation, whilst others have squamous and ductal features not apparent histologically [15, 118], and occasionally both patterns are evident in the same tumours. Some neoplasms

Number of tumours

called small cell carcinoma may in fact be primary primitive neuroectodermal tumours [44]. They are seen more often in men, and the mean age is 56 years (range 5-86) [62]. The microscopic appearance may be similar to small cell carcinoma of the lung or Merkel cell carcinoma of the skin. Both comprise solid sheets, nests and cords of closely packed cells; the difference is in the cell size - small and dark cells in the former, slightly larger and with pale chromatin in the latter.

Immunohistochemistry shows positive staining for chromogranin, synaptophysin, neuron-specific eno-lase and CAM5.2, often with paranuclear dots in both types. However, immunohistochemistry for cytokera-tin 20 seems to identify two subtypes of small cell carcinoma: CK 20- lung cell type and CK 20+ Merkel cell type carcinoma. A recent study by Nagao et al. [148] showed that CK 20+ small cell carcinomas of the salivary glands have a better prognosis than CK 20- cases. This suggests that staining for CK 20 should be performed in SCC as the results may have prognostic value [148]. The differential diagnosis includes metastasis from small cell carcinomas of the lung and this must be excluded before a primary small cell carcinoma can be said to be of salivary origin. Lymphomas and primary primitive neuroectodermal tumours of the salivary glands [44, 105] may be somewhat similar morphologically, and can be excluded immunohis-tochemically.

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