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 fndings 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 specifcity of 99%. 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.[10,11,12] 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.
Dermoid. (A) This transvaginal ultrasound shows a heterogeneous echogenic adnexal mass with areas of increased echogenicity (arrows). (B) This T1-weighted (T1W) image shows a left adnexal mass (arrows) with areas of increased signal. (C) This precontrast fat-suppressed 3-dimensional gradient-recalled echo (GRE) image shows fat suppression of high T1 signal that is consistent with a fat-containing lesion (arrows). (D) There is no discernible enhancement on this T1W postcontrast GRE image. The adjacent myometrium is enhanced (arrowhead).
The most common subtypes of sexcord stromal tumors are fbromas, fbrothecomas, thecomas, and Brenner tumors. Although these account for only 4% of all ovarian tumors, they are the most common solid primary tumor of the ovary. These benign lesions are composed of dense fbrous tissue. On ultrasound, they appear as solid hypoechoic masses exhibiting marked attenuation. It is often diffcult to distinguish fbromas and fbrothecomas from other solid ovarian masses and pedunculated fbroids on ultrasound. On MRI, they exhibit low signal intensity on T2W images with low-to-intermediate signal on T1W images (Figure 5). Small areas of cystic degeneration and edema may be present in larger lesions, and they tend to show mild enhancement with gadolinium contrast. Although similar in MRI appearance to uterine leiomyomas, their ovarian origin can be confrmed by the presence of follicles in the surrounding ovarian tissue. MRI can also exclude these lesions by identifying the ovaries as separate from the lesion.
Fibroma. (A) This transvaginal ultrasound shows a solid adnexal mass (arrows) adjacent to the uterus (arrowhead). (B) An axial T2- weighted fast spin-echo image shows a low-T2-signal mass (arrows) in the left adnexa. (C) Another precontrast 3D gradient-recalled echo (GRE) image reveals the mass (arrows) as isointense to muscle. (D) This postcontrast 3D GRE image shows the mass enhancing (arrows) less than muscle.
Appl Radiol. 2008;37(1):9-24. © 2008 Anderson Publishing, Ltd.
Cite this: Imaging the Female Pelvis: When Should MRI be Considered? - Medscape - Jan 01, 2008.