Case 23 - Discussion
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.