This is a classic case of dermoid cyst (teratodermoid, mature teratoma), by US (unavialable images) we suspect a missed surgical towel regarding the patient history however we did not exclude ovarian teratoma.

We recommended complementary CT scanning.

Dermoid cyst is a term used almost always interchangeably with teratomas.

Dermoids are tumors derived from more than one germ layers, usually the three (ectoderm, mesoderm and endoderm).