Hybrid cyst: a combined epidermoid and trichilemmal cyst.

Seven cases of hybrid cyst, a combined epidermoid and trichilemmal cyst, are reported. Hybrid cysts occurred in five men and two women, and involved the scalp, cheek, back, arm, elbow, and forearm. The clinical diagnosis was usually cyst or sebaceous cyst. Microscopically, the upper portion of the cyst showed epidermoid keratinization in continuity with the surface epidermis, and was indistinguishable from an epidermoid cyst. Showing sharp transition from it, the inferior portion of the lesion displayed trichilemmal keratinization and was indistinguishable from a trichilemmal cyst. Hybrid cyst is not merely a newly described pathologic curiosity; rather, it forms a meaningful link between the two most common types of follicular cysts and adds weight to the body of evidence that most epidermoid cysts are related to the follicular infundibulum, while trichilemmal cysts are related to the follicular isthmus.