Dear Dr. Eva,

What is a cyst? I have ovarian cysts. My partner had a ganglion cyst taken off her foot.

My mother has cysts on her scalp. A friend has cysts in her breasts. Are these all the same thing? What causes them? Do they ever turn into cancer?

Plain Jane


Dear Jane,

“Cyst” is a general term, like “rash” or pain,” that is used for many different conditions. A cyst is any small, closed, rounded area, anywhere in the body, that has a lining and is filled with material different from the normal tissue around it. A cyst can be in skin, in bone, in solid tissues such as the kidney, ovary, breast, liver, spleen, thyroid or adrenal glands, or in air-filled organs like the sinuses and lungs. Most cysts are filled either with thin fluid or thick, bad-smelling waxy material. Although cysts can develop within cancers, the non-cancerous cysts discussed in this article do not become cancerous.

Cysts that are on or inside the skin, or just below the skin, can be seen or felt. Cysts on internal organs and bone are found on diagnostic imaging, by x-ray, sonogram, mammogram, CT scan, or MRI. Most cysts found on imaging are what doctors call “incidentalomas,” meaning they are found accidentally during a test done to look for something else, and the cysts have no medical importance.

Most cysts develop during adulthood. Some, however, are formed before birth, during development of the embryo and fetus. Most of these are simple cysts of the liver or kidneys. Dermoid cysts are an unusual type of developmental cyst that can include several types of tissue, including teeth and hair.

Cysts are classified as “simple” or “complex.” A simple cyst is a pocket of fluid or air. Complex cysts may include more than one type of material, for example part liquid and part air. Complex cysts may have internal walls, called ‘septa,’, that divide the cyst into two or more sections. Some types of complex cysts can be cancerous, so it is often recommended that they be sampled (biopsied) or removed.

Epidermal cysts, also called sebaceous or epidermoid cysts, and called trichilemmal or pilar cysts when on the scalp, are the commonest type of cyst. They can develop anywhere on the skin, but most often on the trunk or scalp. Epidermal cysts form when a small part of the epidermis, the outermost layer of the skin, grows into the dermis, the inner part of the skin, forming a pocket. Epidermal cells make keratin, a thick protein that coats the skin and forms the hair and nails. As the cells of the epidermal pocket continue to secrete keratin, the growing collection of keratin forms a cyst. Epidermal cysts are typically less than an inch across, but over years they can grow much larger. Epidermal cysts can become infected when bacteria from the skin enter the cyst. An infected cyst becomes tender and red and may drain on its own, or heal from inside without draining. If an infected cyst does not improve, surgical drainage may be needed. When there is no infection, it’s not helpful to drain epidermal cysts. This is because the ingrown epidermal pocket is still present after drainage, and will produce more keratin to refill the cyst. To get rid of an epidermal cyst, minor surgery must be done to remove the lining of the cyst as well as the keratin.

Ganglion cysts are most often found on the hand, wrist or foot. They form on the surfaces of ligaments, tendon sheaths and joint linings, and are filled with the same fluid these tissues normally produce to lubricate the joints. Ganglion cysts can change size rapidly, enlarging with increased use of the underlying joint and decreasing in size with less use of the joint. Like sebaceous cysts, draining the fluid does not get rid of the cyst because the fluid will build up again. Rarely, ganglion cysts must be surgically removed when they interfere with use of the hand or wrist or make it uncomfortable to wear shoes.

Cysts form on the ovaries as part of normal ovulation, the process of release of an egg from the ovary, which occurs monthly in most women of reproductive age. A cyst, called a “follicle,” forms around each egg before it is released. Usually, the follicle is absorbed into the ovary after ovulation. Sometimes, the follicle grows larger instead of being absorbed, becoming an ovarian cyst. Rupture of a large ovarian cyst can be painful, but the ruptured cyst will then resorb and only pain management is needed, not surgical intervention.

Breast cysts are pockets of gland tissue in women’s breasts which respond to hormones, swelling and becoming tender in the week before menstruation. This is a normal process and does not require intervention. Decreasing caffeine intake can help reduce the tenderness.

Although it is rare in developed countries, parasites such as trichinosis, tapeworms, and toxoplasma can form cysts in the muscles, liver, brain, lungs, and eyes of people and animals. When cysts are caused by parasites, the parasite or its eggs may be visible inside the cyst on imaging. These cysts must be surgically removed to remove the parasite.

So, cysts can be many different things! If you or someone you care for has a cyst, you have to find out first what kind of cyst they have to learn what, if anything, needs to be done about it. 

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Dr. Eva Hersh, MD
Dr. Eva Hersh, MD
Eva Hersh is a family physician. Send your comments and questions to her by email at