A trichilemmal cyst, also called a pilar cyst, is a non-cancerous growth that develops from a hair follicle in the skin. These cysts are very common and most often appear on the scalp, where they look like smooth, round bumps under the skin.
Trichilemmal cysts can range in size from a few millimeters to several centimetres. They are usually painless, but they can sometimes become tender if irritated, infected, or if they rupture (break open).
Most trichilemmal cysts do not cause symptoms. People usually notice them as a firm, round bump under the skin of the scalp.
When symptoms do occur, they may include:
Tenderness or pain if the cyst becomes inflamed or infected.
Redness or swelling around the cyst if it ruptures.
Rapid growth, which can sometimes cause concern, does not mean the cyst is cancerous.
Trichilemmal cysts develop from the outer root sheath of a hair follicle, which is the part of the skin that produces hair. For reasons that are not fully understood, the cells lining the follicle begin to grow abnormally and form a sac filled with keratin (the protein that makes up hair and skin).
Factors that may contribute to trichilemmal cysts include:
Genetics: They often run in families, suggesting an inherited tendency.
Hair follicle blockage: Trauma or irritation to the hair follicle may play a role.
Age: They are more common in middle-aged and older adults.
Trichilemmal cysts can occur in people of all ages, but they are most often seen in middle-aged adults. They tend to run in families, which suggests a possible genetic predisposition for some individuals to develop them. Women are slightly more likely than men to have trichilemmal cysts.
Doctors often suspect a trichilemmal cyst based on its appearance and location (a smooth bump on the scalp).
The diagnosis can be confirmed in two ways:
Biopsy: A small tissue sample is removed and examined under the microscope.
Excision: The entire cyst is removed surgically and sent to a pathologist.
Once the cyst is entirely removed, no further treatment is usually needed.
When examined under the microscope, trichilemmal cysts have some unique features:
The cyst is filled with keratin (the protein that makes up hair and skin), which looks compact and smooth. Pathologists sometimes describe this as “wet” keratin.
The keratin is surrounded by a wall made of keratinocytes (skin cells). Unlike normal skin, this wall does not have a granular layer.
Sometimes trichilemmal cysts can rupture (break open). When this happens, the keratin spills into the surrounding tissue. The body responds to this by sending a large number of immune cells, which surround the cyst and cause redness, swelling, or pain.
Your pathologist will carefully examine the tissue to make sure there are no signs of cancer, as rare tumours can sometimes mimic a cyst.
For most people, trichilemmal cysts are harmless and typically do not recur after they are entirely removed. If multiple cysts develop, they can be removed individually if they cause discomfort or cosmetic concern.
What are the chances the cyst will come back after removal?