Pilar cyst

by Pavandeep Gill MD FRCPC and Allison Osmond MD FRCPC
May 11, 2023

What is a pilar cyst?

A pilar cyst is a non-cancerous growth that starts from a hair follicle in the skin. They are very common and most start on the scalp where they look like a smooth, round bump on the skin. Pilar cysts range in size from a few millimeters to several centimeters. Another name for a pilar cyst is a trichilemmal cyst.

What causes a pillar cyst?

Most pillar cysts are caused by cells becoming trapped within a hair follicle just below the surface of the skin.

How do pathologists make this diagnosis?

The diagnosis of pilar cyst may be made after a small sample of tissue is removed in a procedure called a biopsy. The diagnosis can also be made after the entire cyst is removed and sent to a pathologist for examination. No further treatment is usually required after the entire cyst has been removed.

What does a pillar cyst look like under the microscope?

When viewed through the microscope, most pilar cysts are filled with compact, ‘wet’ looking keratin. The keratin is surrounded by a layer of flattened keratinocytes without the usual granular layer. Pilar cysts can rupture which means they break open and spill their keratin into the surrounding normal tissue. When this happens, your pathologist may see a large number of immune cells surrounding the cyst.