A sebaceous cyst is a term that loosely refers to either epidermoid cysts (also known as epidermal cysts; L72.0) or pilar cysts (also known as trichilemmal cysts; L72.1). Because an epidermoid cyst originates in the epidermis and a pilar cyst originates from hair follicles, by definition, neither type of cyst is strictly a sebaceous cyst. The name is regarded as a misnomer as the fatty, white, semi-solid material in both of these cyst entities is not sebum, but keratin. Furthermore, under the microscope neither entity contains sebaceous glands. In practice, however, the terms are often used interchangeably.
The scalp, ears, back, face, and upper arm, are common sites for sebaceous cysts, though they may occur anywhere on the body except the palms of the hands and soles of the feet. In males a common place for them to develop is the scrotum and chest. They are more common in hairier areas, where in cases of long duration they could result in hair loss on the skin surface immediately above the cyst. They are smooth to the touch, vary in size, and are generally round in shape.
They are generally mobile masses that can consist of:
- Fibrous tissues and fluids,
- A fatty (keratinous) substance that resembles cottage cheese, in which case the cyst may be called "keratin cyst" This material has a characteristic "cheesy" or "foot odor" smell,
- A somewhat viscous, serosanguineous fluid (containing purulent and bloody material).
The nature of the contents of a sebaceous cyst, and of its surrounding capsule, will be determined by whether the cyst has ever been infected.
With surgery, a cyst can usually be excised in its entirety. Poor surgical technique or previous infection leading to scarring and tethering of the cyst to the surrounding tissue may lead to rupture during excision and removal. A completely removed cyst will not recur, though if the patient has a predisposition to cyst formation, further cysts may develop in the same general area.