Trichilemmal Cysts
Trichilemmal or pilar cysts are common intradermal or subcutaneous cysts, occurring in 5-10% of the population. Greater than 90% occur on the scalp, where trichilemmal cysts are the most common cutaneous cyst. Trichilemmal cysts are almost always benign. They may be sporadic or they may be autosomal dominantly inherited. They contain keratin and its breakdown products and are lined by walls resembling the external (outer) root sheath of the hair. In 2% of trichilemmal cysts, single or multiple foci of proliferating cells lead to tumors called proliferating trichilemmal cysts. Proliferating trichilemmal cysts are gradually enlarging (up to 25 cm in diameter), exophytic nodules that occasionally ulcerate. Although biologically benign tumors, they may be locally aggressive. Recurrences and metastases have been observed, with rare malignant transformation