Case Report: A rapidly growing cyst on the scalp.

Trichilemmal carcinoma is a rare tumour derived from the outer root sheath of hair follicles .  It can be difficult to distinguish both clinically and histologically from other skin lesions, particularly squamous cell carcinoma.  We present the case of a 62-year-old female with a 20-year history of three 1-cm cysts on her scalp.  Over a six-month period, a cyst overlying the occiput had become painful and grown in size.  The general practitioner and subsequently local emergency department suspected infection.  The lesion was incised, and the patient was treated with oral antibiotics.  At the time of surgical excision, the lesion measured 3 x 4 cm. Microscopic examination identified rounded dermal lobules of squamous epithelium with trichilemmal keratinization, in keeping with a pre-existing pilar cyst.  There were areas with nuclear pleomorphism, mitoses and an infiltrative architecture.  A diagnosis of trichilemmal carcinoma arising in a pilar cyst was made.  Trichilemmal carcinomas are considered to be a low-grade tumour, but they have the potential to spread to lymph nodes and to metastasise to distant sites in the body, therefore adequate excision and appropriate follow-up are required.