Pathologic Quiz Case: A Nodule on the Back

An 89-year-old woman presented with a recurrent, painless nodule on her back, which was diagnosed as a proliferating trichilemmal tumor after excision. This uncommon but usually benign lesion is often misdiagnosed as squamous cell carcinoma. It typically occurs on the scalp, but can appear on other body parts, and is more common in women aged 27 to 83. The tumor arises from the outer sheath of the hair follicle and is characterized by sharply circumscribed lobules of squamous epithelium with trichilemmal keratinization. Differentiation from squamous cell carcinoma is crucial, as proliferating trichilemmal tumors have minimal pleomorphism and low mitotic activity. Although generally benign, malignant transformation can occur, 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.