Pathology of Trichilemmoma - Dr Sampurna Roy MD

Syn: Tricholemmoma.

Trichilemmoma arises from the outer root sheath of the hair follicle (mainly of the bulb region).

Trichilemmoma was first described by Headington and French in 1962, as a clear cell tumor with differentiation towards the outer hair root sheath.

This tumour may be associated with Cowden's syndrome.

[ Cowden's syndrome (multiple hamartoma syndrome) is an autosomal dominant disease characterized by multiple cutaneous hamartoma (trichilemmoma, fibroma, verrucous lesions), visceral hamartoma (hyperplastic gastric polyp) or visceral carcinoma (breast carcinoma). ]

Clinical presentation : Solitary or multiple papules in adults ; dome shaped, flesh coloured lesions usually less than 5mm in diameter.

Sites : Face, nose, eyelids, lips and oral cavity.

Microscopic features :

Symmetrical tumour ;

Well- circumscribed margin resting on PAS-positive thickened basement membrane ;

Composed of glycogenated clear epithelial cells with peripheral palisading in deeper parts ;

Cells are PAS- diastase positive ;

Some lesions contain eosinophilic "intermediate cells" (some times known as follicular poroma) ;

Architectural patterns ranges from follicle like, vertically orientated, bulbous, lobular, verrucous or poroma- like ;

There is broad connection with overlying surface epithelium ;

Some cases display connection with individual hair follicle ;

Epidermal changes resembling verruca vulgaris present in some cases (focal koilocytosis and prominent hyaline granules).

Diagnostic feature of trichilemmoma: Evidence of outer root sheath differentiation characterized by:

1. Bland epithelial cells showing peripheral palisading ;

2. Clear cytoplasm ;

3. Prominent intercellular borders ;

4. Thickened and eosinophilic, PAS- positive basement membrane.

These changes are usually noted at the base or at the lateral margin of the lesion.

Immunohistochemistry: CD34 and cytokeratins: Positive

Differential diagnosis:

Trichilemmal Carcinoma ; Basal cell carcinoma ; squamous cell carcinoma,

Hidradenoma; pilar tumour and other adnexal tumours showing focal trichilemmal differentiation.

Clear cell poroma (contain duct lumina and there is no basement membrane or peripheral palisading).

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