Pathology of Proliferating Trichilemmal Tumour

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Dermpath-India

Dr Sampurna Roy MD 2022

Dermatopathology Quiz Case: 262

Answer -Proliferating Trichilemmal Tumour

Trichilemmal (pilar) cysts are common skin lesions that usually occur on the scalp of elderly women.

They differentiate towards the follicular outer root sheath epithelium and show trichilemmal keratinization.

Proliferating trichilemmal tumour shows features of typical pilar cyst, but additionally shows extensive epithelial proliferation, variable cytologic atypia and mitotic activity.

Proliferating trichilemmal tumour arises from the isthmus region of the outer root sheath.

Clinical presentation: Usually occurs in elderly women and presents as a slowly enlarging, painful subcutaneous scalp nodule.

Sites: Scalp is the most common site. Other sites include forehead, nose, back, chest, abdomen, buttocks, elbow, wrist, and vulva .

Gross: Well defined and lobulated lesion (0.4 to 10 cm in diameter).

Microscopic features:

Well-circumscribed lesions with islands of squamous epithelium displaying trichilemmal keratinization.

Small cystic areas may be present.

Foci of calcification, glycogen rich clear cells and horn pearls or squamous eddies may be present.

Mild degree of pleomorphism is common.

Differential diagnosis :

Squamous cell carcinoma - (Proliferating trichilemmal tumour is characterized by abrupt keratinization, minimal pleomorphism, low mitotic activity, sharp circumscription, foci indistinguishable from a trichilemmal cyst, calcification and absence of a premalignant lesion such as actinic keratosis).

The tumour often recurs if not fully excised.

Malignant Proliferating Trichilemmal Tumour:

This malignant tumour originates from a preexisting trichilemmal cyst.

Clinical presentation: Presents as a large ( 20 - 25cm) exophytic lobulated mass usually in elderly women.

Patient complains of sudden rapid growth .

Site: Confined to the scalp, back or neck.

Microscopic features:

- Areas of transformation from benign proliferation of pilar sheath epithelium to an obvious malignant neoplasm ;

- It is characterised by poorly defined borders, clear cut infiltrative properties with cytological evidence of malignancy ;

- Extensive dermal lobular proliferation of pilar sheath epithelium is seen together with palisading arrangement of nuclei at the periphery of lobes ;

- Multiple central areas of trichilemmal keratinization with occasional areas of calcification and homogeneous keratin cyst formation ;

- Epithelium shows severe disarrangement of epithelial cells with large number of cells having irregular shaped hyperchromatic nuclei ;

- There is brisk mitosis ; dyskeratotic cells ; shadow cells and multinucleated cells are also present ;

- Tumour invades into the surrounding connective tissue.

Differential diagnosis: Squamous cell carcinoma - Abrupt mode of keratinization and foci of calcification are characteristic of malignant proliferating trichilemmal tumour.

This low grade malignant tumour shows recurrence following excision and subsequently develops metastasis.

Related posts: Trichilemmal carcinoma ; Trichilemmoma