Definition

 The tumour is composed of columnar epithelial cells arranged in thin, anastomosing cords often with a beaded pattern. The stroma is characteristically paucicellular and highly vascular.

ICD-O code 8149/0

Synonyms

Basal cell adenoma, canalicular type, monomorphic adenoma, canalicular type, adenomatosis of minor salivary glands

Epidemiology

There is a peak incidence in the seventh decade (mean 65 years). The age range is 33-87 years. It is uncommon before the age of 50 {529,668,1864} and the female-to-male ratio is 1.8:1 {529,668}. It comprised 1% of all salivary gland neoplasms and 4% of minor salivary gland neoplasms in a major series {668}.

 Localization

Canalicular adenoma has a peculiar predilection to involve the upper lip (about 80% of tumours) {529,1864}. The next most common location is the buccal mucosa (9.5% of tumours) {1864}. Rarely, canalicular adenoma can involve the major salivary glands {529}.

Clinical features

These tumours present as enlarging nodules with no accompanying symptoms such as pain or paralysis. The overlying mucosa shows typical coloration but in some cases may appear bluish. A peculiar presentation of canalicular adenoma is that of multiple /multifocal canalicular adenomas {1308,1866,2206}. When this occurs, the upper lip and buccal mucosa are typically involved but other sites can be affected.

Macroscopy

Canalicular adenomas range in size from 0.5-2.0 cm in diameter and are grossly well circumscribed. The colour is light yellow to tan {668}.

Histopathology

The microscopic appearance at low magnification likewise shows circumscription. Some canalicular adenomas have a fibrous capsule while smaller tumours often do not. It is not uncommon to see multifocal microscopic canalicular adenomas adjacent to a larger canalicular adenoma. In addition, very small foci of adenomatous tissue can be seen which may represent the earliest recognizable microscopic manifestation of canalicular adenoma. Superimposed necrosis can occur in some cases {36}. The epithelial component manifests as two rows of columnar cells which alternately are situated opposed to each other and alternately widely separated. This leads to the characteristic appearance of these tumours - canaliculi - where the epithelial cells are widely separated. The alternating arrangement of closely opposed and widely separated epithelial cells also leads to the characteristic beaded appearance of these tumours. The epithelial cells forming the cords are typically columnar but can be cuboidal. Nuclei are regular and show no pleomorphism. Nucleoli are inconspicuous and mitotic figures are rare. The stroma is characteristic and a useful clue to the diagnosis. It is paucicellular but shows a prominent vascular pattern. The capillaries often have an eosinophilic cuff of connective tissue.

 Immunoprofile

Canalicular adenomas stain with antikeratin, anti-vimentin and anti S-100 antibodies {758}. Rare focal GFAP positivity is seen {758}. Canalicular adenomas are devoid of staining when more sensitive markers of myogenous differentation such as smooth muscle actin, smooth muscle myosin heavy chain and calponin are used {2883}.

Differential diagnosis

The most important are adenoid cystic carcinoma and basal cell adenoma. Multifocality and cribriform pattern should not be misinterpreted as carcinoma. Hybrid tumours composed of canalicular adenoma and basal cell adenoma have been reported {2297}.

Prognosis and predictive factors

The prognosis is excellent and recurrences are rare even if the tumours are treated with just a local excision or lumpectomy. Whether new tumours are true recurrences or are a manifestation of the multicentric growth pattern is difficult to ascertain.