Pathologic Quiz Case: A Nodule on the Back

An 89-year-old woman presented with a recurrent, painless nodule on her back, initially diagnosed as a sebaceous cyst. Upon excision, it was identified as a proliferating trichilemmal tumor, a typically benign lesion often mistaken for squamous cell carcinoma. These tumors, arising from the hair follicle's outer sheath, are usually found on the scalp and are more common in women aged 27 to 83. Histologically, they are characterized by sharply circumscribed lobules of squamous epithelium with trichilemmal keratinization. Although generally benign, they can occasionally undergo malignant transformation, necessitating surgical excision with clear margins. The case was typical in terms of patient demographics and tumor characteristics, though its location on the back was unusual.