Tumours of germ cell origin are the second most common group of ovarian neoplasms, representing 15%–20% of all ovarian tumours. They are bilateral in 8-15% of cases. This group of tumours includes mature teratoma, immature teratoma, dysgerminoma, endodermal sinus tumour, embryonal carcinoma, and choriocarcinoma) Of all the germ cell tumours, only mature teratoma is benign; however, it is by far the most common lesion in this group. Teratomas comprise a number of histologic types of tumours, all of which contain mature or immature tissues of germ cell (pluripotential) origin. The most common of these tumours, the mature cystic teratoma (also known as dermoid cyst), typically contains mature tissues of ectodermal (skin, brain), mesodermal (muscle, fat), and endodermal (mucinous or ciliated epithelium) origin. Mature cystic teratomas (a more appropriate term than the commonly used "dermoid cysts") are cystic tumours composed of well-differentiated derivations from at least two of the three germ cell layers (ectoderm, mesoderm, and endoderm). They affect a younger age group (mean patient age, 30 years) Mature cystic teratoma can be associated with complications from rupture, malignant degeneration, or (most commonly) torsion. In monodermal teratomas, one of these tissue types (e.g. thyroid tissue in struma ovarii, neuroectodermal tissue in carcinoid tumour) predominates. Ovarian teratomas include mature cystic teratomas (dermoid cysts), immature teratomas, and monodermal teratomas (e.g. struma ovarii, carcinoid tumours, and neural tumours). The morphologic features of the tumours differ in that mature cystic teratomas (dermoid cysts) are predominantly cystic, whereas immature teratomas are predominantly solid with small foci of fat.
At CT, fat attenuation within a cyst with or without calcification in the wall, is diagnostic for mature cystic teratoma. A floating mass of hair can sometimes be identified at the fat–aqueous fluid interface. Fat is reported in 93% of cases and teeth or other calcifications in 56%. A round Rokitansky nodule has a feathery appearance at the fatty interface where the hair arises from it.
At MR imaging, the sebaceous component is specifically identified with fat-saturation techniques. On T1- and T2-weighted images, the cystic spaces demonstrate both high and low signal intensity. At MR imaging, the sebaceous component of dermoid cysts has very high signal intensity on T1-weighted images, similar to retroperitoneal fat. The signal intensity of the sebaceous component on T2-weighted images is variable, usually approximating that of fat. This combination of different signal intensities on T1- and T2-weighted images is not specific for fat and must be distinguished from MR imaging findings in intracystic haemorrhage, which can cause shortened T1 and T2 of the cyst fluid . The imaging appearance on T1- and T2-weighted images is therefore mimicked by some hemorrhagic lesions, most prominently endometriomas. Endometriosis co-existing with bilateral dermoid cysts of the ovaries is a rare occurrence although both benign conditions are said to be common in women in reproductive age group. This association has a clinical relevance because an endometriotic pathology can reveal a silent teratoma with bilateral ovarian localization