Fine-needle aspiration cytology in the diagnosis of malignant proliferating trichilemmal tumor: Report of a case and review of the literature.

Malignant proliferating trichilemmal tumor is a rare cutaneous neoplasm derived from the outer root sheath of the hair follicle. Fine-needle aspiration cytology is being increasingly used in the investigation of primary and metastatic cutaneous tumors. However, there are few reports on the cytology of trichilemmal tumors in the literature. We describe the cytological features of this uncommon adnexal tumor presenting as a scalp mass in a 58-year-old woman. In view of its aggressive biological behavior, it is crucial for cytologists to be aware of this rare lesion and distinguish it from primary cutaneous squamous-cell carcinoma. The differences on fine-needle aspiration cytology can be subtle and pose problems in diagnosis.