Germ cell tumors represent 15% to 20% of all tumors of the ovary. Dermoids account for 95% of all ovarian germ cell tumors. Most of these are unilocular, contain sebaceous fluid, and are commonly referred to as mature cystic teratomas or dermoid cysts. Although these are usually asymptomatic and are incidental findings in young women, the standard treatment is surgical removal because of their potential to cause ovarian torsion or for the cyst to rupture. There is also a rare chance of malignant degeneration to squamous cell carcinoma. Although most mature cystic teratomas can be diagnosed at ultrasound, one prospective study has shown the sensitivity to be 58% with a specificity of 99%.11 Numerous pitfalls exist in their diagnosis by ultrasound. The presence of blood clot within a hemorrhagic cyst can appear echogenic, which causes confusion in the diagnosis. Adjacent echogenic bowel can also be mistaken for a mature cystic teratoma and vice versa.
MRI has a high sensitivity for the presence of fat within the sebaceous component, which is characteristic of nearly all these lesions. The sebaceous component is of very high signal intensity on T1W images and is somewhat variable on T2W images.10-12 Fat suppression can differentiate macroscopic fat from other hemorrhagic lesions that appear hyperintense on T1W images, such as hemorrhagic cysts and endometriomas (Figure 4). Even the rare lesion that contains microscopic fat can be differentiated by using chemical shift imaging with the use of in- and out-of-phase sequences. Mature cystic teratomas also commonly have a solid mural nodule that is referred to as a dermoid plug or a Rokitansky nodule. Although rare, malignant transformation can occur in 1% to 2% of cases. In these cases, the women tend to be postmenopausal and the images are characterized by transmural extension of the solid component and, often, by direct invasion of adjacent pelvic structures.
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