A Case of a Clinically Proliferating Trichilemmal Cyst Following the Take of Febuxostat and Calcium Carbonate

Trichilemmal cysts or pilar cysts are keratin-filled cysts that originate in the outer root sheath. They are typically found on the scalp of middle-aged women. They are inherited as an autosomal dominant trait. Trichilemmal cysts range from simple trichilemmal cysts to benign and malignant proliferative trichilemmal cysts. Proliferation can be clinical or histological. Clinically, it may present as an increase in size with the onset of an ulceration. Histologically, it translates as the acquisition of certain features which are the dermal proliferation of a squamous epithelium that arranges in a lobular fashion with clear cells containing glycogen in some areas surrounded by a somewhat cellular and glassy stroma; and especially the trichilemmal keratinization in the centre of lobules. There are also certain histopathological criteria to distinguish benign from malignant proliferating trichilemmal cysts. A malignant trichilemmal cyst is characterized by a high mitotic index, atypical mitoses, significant nuclear polymorphism, and tumor invasion of adjacent tissues. Immunohistochemistry can also confirm the benign character by expression of cytokeratin 10 and involucrin. We report here a case of a clinically proliferating trichilemmal cyst following the introduction of calcium carbonate and febuxostat in a patient undergoing treatment for hypertension and renal failure, but retaining a typically simple and benign character on histology.