Pathology of Trichilemmal Carcinoma

Trichilemmal carcinoma was originally described by Headington as a "histologically invasive, cytologically atypical clear cell neoplasm of adnexal keratinocytes which is in continuity with the epidermis and / or follicular epithelium."

The outer root sheath consists of cells with clear vacuolated cytoplasm due to presence of abundant glycogen.

The trichilemmal carcinoma is a malignant tumour of the outer hair sheath.

Clinical presentation:

A slow growing tumour which shows predilection for sun-exposed hair bearing skin.

Clinically, the tumour presents as a pale tan or reddish papule, indurated plaque or nodule.

The tumour size is usually between 0.4 and 2.0 cms.

Sites: The usual sites include scalp, face, trunk, upper extremities.

Microscopic features:

- Tumour shows wide range of growth patterns (solid, lobular, trabecular) ;

- Tumour lobules are sharply circumscribed by a hyaline PAS (+) ve membrane, and infiltrates with a pushing border ;

- Lobules consist of large tumour cells with PAS reactive and diastase sensitive clear or pale eosinophilic cytoplasm ; stain for mucin is negative ;

- There are foci of pilar-type keratinization and peripheral palisading of cells with subnuclear vacuolization ;

- Some dyskeratotic cells and numerous mitotic figures are present ;

- There is actinic damage ; superficial ulceration;

- In some cases tumour shows pagetoid spread ;

- Others infiltrate into the deep dermis.

Immunohistochemistry:

There is positivity for cytokeratin and negativity for CEA and EMA.

Clinical course:

Despite an aggressive growth it has an indolent clinical course.

No cases with recurrence or metastasis have been reported in the literature.

Differential diagnosis:

Clear cell carcinomas of the skin.

Benign tumours like trichilemmoma and hidradenoma lack the infiltrative growth pattern and cytological atypia.

Trichilemmal carcinoma and malignant proliferating trichilemmal tumour are two separate entities.

The latter, usually arises in a preexisting trichilemmal cyst and has metastatic potential.