Ovarian and paraovarian squamous-lined cysts (epidermoid cysts): a clinicopathologic study of 18 cases with comparison to mature cystic teratomas.

The origin of ovarian epidermoid cyst is unknown. Sixteen ovarian and 2 paraovarian squamous-lined cysts unassociated with teratomatous elements were studied. The ovarian cysts represented 1.5% of consecutive ovarian surface epithelial tumors examined and were one-nineteenth as common as mature cystic teratoma. The mean patient age was 57 years. All tumors were unilateral and the majority were incidental findings. The mean tumor size was 1.75 cm and the median, 3.0 cm. In comparison to 120 consecutive patients with mature cystic teratoma who had a mean age of 41 years and a mean tumor size of 6 cm, the ovarian epidermoid cysts were significantly smaller and occurred at a significantly older age (P<0.01). All cysts displayed mature squamous epithelium with a granular layer, with hyperkeratosis in 14 and parakeratosis in 4. In 2 patients, there were contralateral mature cystic teratomas, and in 2 others the cysts contained rare hairs. Two displayed foci of Brenner tumor and 1 appeared to arise in endometriosis. In summary, 7 of 16 ovarian epidermoid cysts displayed features suggesting they reflected insufficiently sampled teratomas, Brenner tumors with squamous metaplasia, or a metaplastic change in endometriosis. Epidermoid cyst of the ovary as defined by histology is a heterogeneous group; pure epidermoid cyst, if it exists at all, probably represents less than 1% of ovarian surface epithelial tumors.