Trichilemmoma is a benign skin tumour thought to arise from the outer root sheath of the hair follicle.
Histology of trichilemmoma
Trichilemmoma is usually a symmetrical epithelial nodular proliferation. Figure 1. There may be mild papillomatosis with overlying hyperkeratosis. Figure 2. The key finding is of a downgrowth of epithelial cells with increasing clear cell differentiation. Figure 2, Figure 3. These changes are frequently more obvious towards the base of the lesion. The clear cell is PAS positive but diastase labile indicative of the glycogen contents. There is often basal peripheral palisading, resting on a distinctive PASD positive eosinophilic hyaline basement membrane. Figure 4.
Histological variants of trichilemmoma
Desmoplastic trichilemmoma: In this variant the tumour develops infiltrating cords predominantly at the base of the lesion. This is surrounded by a sclerotic stroma and a mild to moderate lymphocytic inflammatory response.
Differential diagnosis of trichilemmoma
- Clear cell basal cell carcinoma: Lacks the eosinophilic basement membrane, and displays mitotic activity often set in a mucinous stroma.
- Hidroacanthoma simplex/eccrine poroma: Demonstrates duct formation
- Trichilemmal carcinoma
- Tumour of the follicular infundibulum
- Desmoplastic trichilemmoma must be differentiated from infiltrating basal cell carcinoma and invasive squamous carcinoma.