Histopathology

Tumours consist of two main cell types: ductal and modified myoepithelial cells that typically have hyperchromatic, angular nuclei and frequently clear cytoplasm. There are three defined patterns: tubular, cribriform and solid. In the tubular form, well-formed ducts and tubules with central lumina are lined by inner epithelial and outer myoepithelial cells.

The cribriform pattern, the most frequent, is characterized by nests of cells with cylindromatous microcystic spaces. These are filled with hyaline or basophilic mucoid material. The solid or basaloid type is formed of sheets of uniform basa-loid cells lacking tubular or microcystic formation. In the cribriform and solid variants small true ducts are invariably present but may not be immediately apparent. Each of these forms can be observed as the dominant component or more commonly as a part of a composite tumour {161,1663,1849,2016,2444, 2519}. The stroma within the tumour is generally hyalinized and may manifest mucinous or myxoid features. In some tumours there is extensive stromal hyalin-ization with attenuation of the epithelial component. Perineural and to a lesser extent, intraneural invasion is a common and frequently conspicuous feature of AdCC. Tumours can extend along nerves for a considerable distance beyond the clinically apparent boundaries of the tumour. In addition, the tumour may invade bone extensively before there is

Fig. 5.7 Adenoid cystic carcinoma. Cribriform pattern with mucopolysaccharide filled spaces.
Table 5.2 Differential diagnosis of adenoid cystic carcinoma

Tumour type

Pattern

Cellular features

Perineural invasion

Basal cell adenoma

Syncytial/ non-invasive

Uniform Basaloid

No

Epithelial-myoepithelial carcinoma

Tubular/biphasic

Uniform, with clear outer cells

Rare

Basaloid squamous cell carcinoma

Syncytial

Marked pleomorphism focal keratinization

Rare

Basal cell adenocarcinoma

Syncytial/invasive

Mild pleomorphism/ invasive

Yes

AdCC Solid

Syncytial

Mild pleomorphism

Yes

cylindromatous

Uniform biphasic Mild pleomorphism

Yes Yes

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