Non-salivary-type low-grade sinonasal adenocarcinomas are a distinctive group of tumours comprising well-differentiated tubular or papillary structures, or a combination of both. They are lined by a single layer of cuboidal or columnar cells that display uniform round or oval nuclei, inconspicuous nucleoli, minimal cytologic atypia and scarce mitotic figures. They are locally infiltrative and have a tendency towards local recurrence .
Different histological patterns may be recognised: papillary, glandular, mucinous, trabecular, cribriform and clear cell. The papillary pattern is characterised by complex papillary fronds lined by bland columnar cells that may occasionally mimic oncocytic (columnar) cell papilloma. Quite similar tumours also develop in the nasopharynx . The glandular pattern may simulate adenoma; nevertheless, the presence of closely packed glands, forming back-to-back arrangements, indicates the true malignant nature. Mucinous tumours have to be distinguished from mucoceles and from mucinous adenocarcinoma of intestinal or salivary type [20, 224]. The trabecular pattern may resemble acinic cell carcinoma . The cribriform arrangements have to be distinguished from low-grade salivary duct carcinoma of salivary glands . The clear cell type has to be separated from the salivary-type tumours with clear cells and
from metastatic renal cell carcinoma [31, 280]. A tubulopapillary variant has recently been reported (Fig. 2.15)  that has to be differentiated from terminal tubulus adenocarcinoma of the nasal seromucous glands .
Muco epidermoid carcinoma, polymorphous low-grade adenocarcinoma (Fig. 2.16) and acinic cell carcinoma originate only on rare occasions from the seromucous glands of the sinonasal mucosa [43, 110, 150, 190, 200, 239]. Most mucoepidermoid carcinomas of the sinona-sal tract are low-grade. Some large oncocytic tumours of the sinonasal tract may behave in a locally aggressive fashion and are better classified as low-grade adenocar-cinomas [55, 103, 110]. All these tumours are dealt with in detail in Chap. 5 on salivary glands. Their main differential diagnoses are other salivary- or non-salivary-type low-grade adenocarcinomas.
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