Dermoid cysts make up about 1 in 8 to 1 in 10 ovarian tumours. They are almost always benign (non-cancerous) with only about 1 in every 100 showing any signs of cancerous changes.

Dermoid cysts are smooth, thick-walled swellings that can be quite large, growing up to 30cm (12 inches) across. They are quite strange tumours in that the cells that form them can produce a variety of different types of tissue, so as well as just fluid the cysts often contain hair and traces of tissue like cartilage, teeth, thyroid gland and bowel tissue. In one variant of the dermoid cyst the tissue within the cyst is almost entirely thyroid tissue, this is called a struma ovarii.

The cysts are lined with a type of cell called squamous epithelium, which are the same cells that form the surface layers of the skin. This is why, historically, these growths have been called 'dermoid' (or 'skin-like') cysts.

As already mentioned finding any cancerous changes in these cysts is very uncommon, occurring about 1%. When this does happen it almost always affects the squamous lining tissues and so the cancer that develops is a squamous cell carcinoma.

Usually all that is required by way of treatment is an operation to remove the cystic growth. This can usually be done as a 'cystectomy' which preserves the underlying ovary and only if the cysts is very large will the ovary need to be removed. In about 12% of cases both ovaries will have cysts and when the operation is done both ovaries will be checked.

As the ovaries are normally spared by the surgery there are no hormonal changes afterwards and, in younger women, periods and fertility will not be affected. The operation usually results in a complete cure.