A Rare Presentation of a Pilar (Trichilemmal) Cyst of the Upper Arm in an Elderly Male.

Pilar cysts, also known as trichilemmal cysts, are benign keratin-filled lesions that arise from the outer root sheath of the hair follicle. Pilar cysts account for a small minority of cutaneous cysts and are most commonly found on the scalp, particularly in middle-aged women. Because they overwhelmingly occur on the scalp, presentation in other anatomical locations is uncommon and can create diagnostic uncertainty. We present the case of an 81-year-old male with a slowly enlarging, painless mass on the left upper arm that had been present for nearly eight years. The lesion was well-circumscribed and firm on examination, without associated erythema, drainage, or tenderness. Given its gradual growth, surgical excision was performed. Grossly, the mass was encapsulated and measured approximately 1.5 cm in diameter. Histopathologic evaluation demonstrated a keratin-filled cyst lined by stratified squamous epithelium with abrupt trichilemmal keratinization and absence of a granular layer, confirming the diagnosis of a benign pilar cyst. There was no evidence of proliferative or malignant transformation. The patient tolerated the procedure well and had an uncomplicated postoperative course. Although pilar cysts are benign, their presence in atypical locations such as the upper extremity may broaden the differential diagnosis to include epidermoid cysts, lipomas, and other soft tissue neoplasms. This case highlights the importance of maintaining clinical suspicion for pilar cysts outside of the scalp and emphasizes the role of histopathologic confirmation in establishing an accurate diagnosis. Recognizing these rare presentations can help prevent misdiagnosis and ensure appropriate surgical management.