A closed sac filled with oily or fatty fluid and debris; also known as a sebaceous cyst. Epidermal cysts commonly develop on the face, neck, upper chest, and back. They are usually benign (not cancerous) and do not require treatment. However, in some cases, epidermal cysts grow large and painful or become infected. Treatment is with antibiotics and, as necessary, surgical incision and drainage or surgical excision (cutting out the cyst).