Trichilemmal cyst

by Pavandeep Gill MD and Allison Osmond, MD FRCPC
November 2, 2022

What is a trichilemmal cyst?

A trichilemmal 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. Trichilemmal cysts range in size from a few millimeters to several centimeters. Another name for a trichilemmal cyst is a pilar cyst.

How is this diagnosis made?

The diagnosis of a trichilemmal cyst may be made after a small tissue sample 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 trichilemmal cyst look like under a microscope?

When viewed through the microscope, most trichilemmal cysts are filled with compact, ‘wet’ looking keratin. The keratin is surrounded by a layer of flattened keratinocytes without the usual granular layer.

Trichilemmal 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. Your pathologist will examine your tissue carefully to make sure there is no sign of cancer.

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