Trichilemmoma

• The epidermis usually reveals hyperkeratosis, mild acanthosis, and, occasionally, a cutaneous horn;

• A lobular or platelike epidermal growth projecting downward into the dermis;

• Cells high in the epidermis may be vacuolated and contain coarse keratohyaline granules; 

• Cells located toward the center of the lobular growth are often pale staining or have clear cytoplasm. These cells contain glycogen and are periodic acid-Schiff (PAS) positive but diastase labile;

• Cells at the periphery of the tumor are more basophilic and exhibit palisading;

• An eosinophilic hyaline basement membrane zone surrounds the tumor. This eosinophilic rim is PAS positive and diastase resistant.

Foci of epidermal keratinization with formation of squamous eddies may be present.

The histologic differential diagnosis includes a hidroacanthoma simplex, which usually demonstrates ductal differentiation. A clear, basal cell carcinoma should also be considered but excluded based on the presence of mucinous stroma, mitoses, and if it demonstrates peripheral palisading but lacks the eosinophilic hyaline cuticle. Other epithelial neoplasms, such as the tumor of the follicular infundibulum, may be considered in the differential diagnosis. Trichilemmal carcinoma may be considered but excluded based on its invasive growth pattern and many mitoses.

A desmoplastic trichilemmoma is considered a histologic variant of the trichilemmoma. It is a benign neoplasm of the hair follicle that is derived from the outer root sheath or infundibular epithelium. This tumor demonstrates a biphasic growth pattern having features of a lobulated tumor and cells that form narrow, irregular cords, which penetrate into the dermis. The stroma surrounding the tumor cords appears sclerotic. An inflammatory cell infiltrate is often seen surrounding the epithelial strands of the tumor. Because of these features, differentiating this tumor from an invasive squamous cell carcinoma or a sclerosing basal cell carcinoma is important. Note that desmoplastic trichilemmoma has not been associated with Cowden disease.