Mucinous adenocarcinoma

Definition

Mucinous adenocarcinoma is a rare malignant tumour composed of epithelial clusters within large pools of extracellular mucin. The mucin component usually occupies the bulk of the tumour mass.

ICD-O code 8480/3

 Epidemiology

 It usually arises in patients over 50 years of age. Males are affected more frequently than females {859,1374,1909, 1957,2551}.

Localization

The most frequently affected sites are the palate and the sublingual gland, followed by the submandibular gland and the upper lip. Occurrence in the parotid gland is rare {859}.

Clinical features

The patients usually present with a slowgrowing, painless swelling. However, local dull pain may be encountered in some cases. The tumour is firm and usually elevated.

Macroscopy

 The tumour is nodular and ill defined. The cut surface is greyish-white, containing many cystic cavities with gelatinous contents.

Histopathology

The tumour is composed of round and irregular-shaped neoplastic epithelial cell nests or clusters floating in mucus-filled cystic cavities separated by connective fibrous strands. The tumour cells are cuboidal, columnar or irregular in shape, usually possess clear cytoplasm and darkly-stained, centrically placed nuclei. The tumour cells may have atypical nuclei, but mitotic figures are sparse. The tumour cells are arranged in solid clusters and tend to form secondary lumens or incomplete duct-like structures. Mucus-producing cells may arrange in a papillary pattern projecting into the mucous pools. Mucous acinus-like tumour islands may also be present. Both intracellular and extracellular mucin components show positive staining for periodic acid Schiff, Alcian blue and mucicarmine.

Immunoprofile

 Immunocytochemically, the tumour cells express pankeratin AE1/AE3 as well as cytokeratins 7, 8, 18 and 19 that are usually found in simple epithelia {859,1374}. Expression of cytokeratins 4 and 13 is seen in about 10-20%. Negative staining is noted for cytokeratins 5/6, 10, 14, 17 and for smooth muscle actin (SMA).

Electron microscopy

The cytoplasm of the tumour cells is densely packed with numerous low-electron- density mucous droplets, and seromucous droplets containing electrondense dots are also seen. Tumour cells possessing mucous or seromucous droplets form a luminal structure, and they have irregularly arranged microvilli on the luminal side.

Differential diagnosis

Mucoepidermoid carcinoma, mucin-rich variant of salivary duct carcinoma and cystadenocarcinoma should be differentiated from mucinous adenocarcinoma. Mucoepidermoid carcinoma also shows extravasated mucin, but it consists of intermediate and epidermoid cells. Cystadenocarcinoma shows cystic spaces lined by epithelium. Extracellular mucin pools are not evident in acinic cell carcinoma.

Prognosis and predictive factors

 Mucinous adenocarcinoma is insensitive to radiotherapy and has a propensity for local recurrence and regional lymph node metastases.