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Atypical CT and MRI Manifestations of Mature Ovarian Cystic Teratomas

¹ Department of Radiology, Kangnam St. Mary's Hospital, College of Medicine, The Catholic University of Korea, 505, Banpo-Dong, Seocho-Ku, Seoul 137-040, South Korea.
² Department of Radiology, St. Mary's Hospital, The Catholic University of Korea, 62 Yeouido-dong, Youngdungpo-gu, Seoul 150-713, South Korea.
³ Department of Radiology, Daejeon St. Mary's Hospital, The Catholic University of Korea, 520-2 Daeheung-dong, Choong-gu, Daejeon 301-723, South Korea.
⁴ Department of Pathology, Kangnam St. Mary's Hospital, College of Medicine, The Catholic University of Korea, Seoul 137-040, South Korea.

Received November 11, 2003; accepted after revision February 13, 2004.

Address correspondence to J. Y. Byun (jybyun{at}catholic.ac.kr).

Mature cystic teratoma is a commonly encountered ovarian tumor, constituting 20% of all ovarian tumors in adults and 50% of all ovarian tumors in children [1]. Mature cystic teratomas are composed of well-differentiated derivations of the three germ cell layers (ectoderm, mesoderm, and endoderm). In most cases, they are easily diagnosed on imaging studies because of their characteristic intratumoral fat component. Although typical imaging findings of mature cystic teratomas are well known to radiologists, various atypical imaging features can be particularly misleading. This article illustrates atypical imaging manifestations of mature ovarian cystic teratomas depending on their tumor components and the presence of combined complications on CT and MRI.

Atypical Imaging Manifestations of Mature Cystic Teratomas Depending on Tumor Components

Mature Cystic Teratomas Without Fat in the Cystic Cavity
A minor percentage of mature cystic teratomas have only a small amount of fat or no visible fat on imaging studies. In one series [2], 15% of mature cystic teratomas did not show fatty tissue in the cystic cavity. Approximately half had only a small fat component in the cystic wall (Figs. 1A and 1B) or in the Rokitansky nodule. In the remainder, fatty tissue was not identified in the lumen or the cystic wall, and the aqueous fluid filled the entire cyst (Figs. 2 and 3A, 3B, 3C).

View larger version (121K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A. —33-year-old woman with mature cystic teratoma without fat in cystic cavity. Axial T1-weighted gradient-echo image (TR/TE, 470/14) shows homogeneous low-signal-intensity mass with focal high signal intensity (arrow) along right-sided anterior wall of mass.

View larger version (144K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B. —33-year-old woman with mature cystic teratoma without fat in cystic cavity. Axial gadolinium-enhanced fat-saturated T1-weighted image (736/14) shows signal suppression (arrow), suggesting presence of small fat component in cyst wall. Fatsaturated MRI or gradient-echo technique with both in-phase and opposed-phase imaging is useful to detect small amount of fatty tissue on MR images.

View larger version (116K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2. —23-year-old woman having atypical mature cystic teratoma with purely cystic component. Axial contrast-enhanced CT scan shows large homogeneous fluid-attenuated mass with focal high calcification (arrowhead) along wall. No detectable fat component is present within cystic mass. In these cases, differentiation from other epithelial ovarian neoplasms may be difficult.

View larger version (124K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3A. —35-year-old woman with atypical mature cystic teratoma without fat component. Axial T1-weighted spin-echo image (TR/TE, 700/17) shows large well-defined low-signal-intensity mass with subtle different signal-intensity lesion (arrowheads) in mass.

View larger version (129K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3B. —35-year-old woman with atypical mature cystic teratoma without fat component. Axial T2-weighted fast spin-echo image (TR/effective TE, 3,200/90) shows mass with homogeneous high signal intensity.

View larger version (131K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3C. —35-year-old woman with atypical mature cystic teratoma without fat component. Gadolinium-enhanced T1-weighted spin-echo image (TR/TE, 500/17) shows no significant contrast enhancement in mass (arrowheads). Pathologically, internal content of mass was cheeselike sebaceous material with less fatty tissue in cyst than in usual cystic teratomas.

Mature Cystic Teratomas with a Pure Fatty Component in the Cyst
Rarely, mature cystic teratomas have a pure fat component on imaging without any other component (Fig. 4). These tumors may mimic other uncommon lipid-containing pelvic tumors such as pedunculated lipomatous uterine tumor, benign pelvic lipoma, and liposarcoma [3].

View larger version (115K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4. —28-year-old woman with mature cystic teratoma composed of pure fat component. Contrast-enhanced CT scan shows well-defined mass of fat attenuation (T) in pelvic cavity. Note absence of detectable calcification or matted tuft of hair in mass. These tumors may mimic other uncommon lipid-containing pelvic tumors. In addition, small-sized teratoma with mainly fatty component and floating debris may be missed on imaging studies or mistaken for gas-filled bowel loops, especially with improper setting of image contrast.

Combination Tumors and Collision Tumors Containing Mature Cystic Teratomas
A combination tumor is defined as a tumor in which the intermixed varying histologic components result from a common stem cell; a mixed germ cell tumor is a typical example. Mixed germ cell tumors contain more than one germ cell component; virtually any combination of cell types can occur among embryonal carcinomas, dysgerminomas, teratomas, and yolk sac tumors (endodermal sinus tumor) [1]. The imaging findings of mixed germ cell tumors are variable and reflect the diversity of this group of tumors. When a predominantly solid and heterogeneous ovarian tumor contains fatty areas or calcifications suggestive of a mature cystic teratoma or when a mature cystic teratoma contains an enhancing solid portion, a diagnosis of a mixed germ cell tumor should be considered (Figs. 5A and 5B).

View larger version (96K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5A. —Mixed germ cell tumors (mature cystic teratoma and yolk sac tumor) in 19-year-old woman with elevated -fetoprotein level of 1,383 ng/mL (normal range, 0–20 ng/mL) and cancer antigen-125 level of 330 IU/mL (normal range, 0–35 IU/mL). Axial contrast-enhanced CT scan shows large cystic mass containing focal fat component (arrowheads), dense calcifications, and several solid enhancing nodules (arrows).

View larger version (134K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5B. —Mixed germ cell tumors (mature cystic teratoma and yolk sac tumor) in 19-year-old woman with elevated -fetoprotein level of 1,383 ng/mL (normal range, 0–20 ng/mL) and cancer antigen-125 level of 330 IU/mL (normal range, 0–35 IU/mL). Photograph of surgical specimen shows multiseptate cystic mass filled with hair and sebum (mature cystic teratoma component) and several solid nodular masses (yolk sac tumor component, arrows). Elevated serum -fetoprotein and human chorionic gonadotropin levels can help establish diagnosis.

Collision tumor is defined as a tumor with two adjacent but histologically distinct tumors without histologic admixture at the interface. Collision tumors involving ovaries are rare, but the most common type of ovarian collision tumor is composed of a mature cystic teratoma and a mucinous cystadenoma or cystadenocarcinoma [4]. Imaging studies of a collision tumor composed of a teratoma and a mucinous tumor show a typical multiloculated cystic mass with an internal locule filled with pure fat (Figs. 6A, 6B, 6C and 7).

View larger version (115K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6A. —36-year-old woman with collision tumor (mature cystic teratoma and mucinous cystadenoma). Axial T1-weighted gradient-echo image (TR/TE, 370/14) shows large multilocular cystic mass with heterogeneous signal intensity in locules. One locule (arrowheads) has very high signal intensity.

View larger version (116K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6B. —36-year-old woman with collision tumor (mature cystic teratoma and mucinous cystadenoma). On axial T2-weighted turbo spin-echo image (3,200/99), mass shows high signal intensity. Central locule (arrowheads) also shows high signal intensity.

View larger version (140K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6C. —36-year-old woman with collision tumor (mature cystic teratoma and mucinous cystadenoma). Gadolinium-enhanced fat-saturated axial T1-weighted gradient-echo image (560/14) shows saturation of high-signal-intensity foci (arrowheads) in mass; finding indicates fat. Surgery revealed collision tumor of right ovary composed of mature cystic teratoma and mucinous cystadenoma.

View larger version (110K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7. —33-year-old woman with collision tumor (mature cystic teratoma and borderline mucinous tumor). Axial contrast-enhanced CT scan shows large multiloculated cystic mass of left ovary with several locules filled with fat (arrows). Pathologically, collision tumor composed of benign cystic teratoma and borderline mucinous tumor was confirmed. Mature cystic teratomas are sometimes incorporated into wall of mucinous cystadenoma.

Atypical Imaging Manifestations of Mature Cystic Teratomas Depending on Combined Complications

Mature cystic teratomas have an unusually high complication rate compared with other ovarian tumors and are prone to torsion, rupture, and infection (often with Salmonella organisms). Malignant transformation is another rare complication. The complicated teratomas show atypical but characteristic imaging findings.

Torsion
Torsion is the most common complication associated with mature cystic teratomas. The rate of torsion was reported at 3.2–16% [5]. Torsion of the ovarian pedicle produces circulatory stasis that is initially venous but becomes arterial. If the torsion is complete, gangrenous and hemorrhagic infarction results. If the torsion is partial and intermittent with spontaneous untwisting, symptoms may subside, only to return within hours, days, or weeks [6].

A torsed teratoma may show atypical imaging findings such as smooth eccentric wall thickening of the mass (Fig. 8), peritumoral infiltration seen as ill-defined linear or reticular shadows in the peritumoral spaces, hemorrhage within the mass, ascites or hemoperitoneum, and a thickened fallopian tube on CT or MRI. A thickened fallopian tube is the most specific imaging finding for adnexal torsion and manifests as an amorphous or tubular masslike structure or has a targetlike appearance between the torsed teratoma and the uterus or a beaklike protrusion extending from the uterus and partially covering the ovarian teratoma [6] (Figs. 9A, 9B and 10A, 10B).

View larger version (99K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8. —Torsion of left ovarian mature cystic teratoma with hemorrhagic infarction in 10-year-old girl with 24-hr history of lower abdomen pain. Contrast-enhanced CT scan shows eccentric smooth wall thickening of mass (arrows) containing fat and teeth. Surgery revealed left ovarian cystic teratoma with torsion of 720° and hemorrhagic infarction.

View larger version (116K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 9A. —Torsion of left ovarian cystic teratoma in 27-year-old woman with 2-day history of lower abdomen pain. Axial gadolinium-enhanced T1-weighted turbo spin-echo image (TR/TE, 500/10) shows ovoid pelvic mass (M) with internal high-signal-intensity fat component. Note amorphous masslike structure (arrows) connecting mass and uterus (U); finding is suggestive of twisted pedicle.

View larger version (139K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 9B. —Torsion of left ovarian cystic teratoma in 27-year-old woman with 2-day history of lower abdomen pain. Sagittal fat-saturated T2-weighted turbo spin-echo image (2,100/80) shows multifocal suppression of high signal intensity (arrowheads) in tumor, suggesting characteristic multiple fat balls in mature cystic teratoma.

View larger version (88K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 10A. —Torsion of right ovarian cystic teratoma in 47-year-old woman with 7-day history of gradually increasing lower abdomen pain. Contrast-enhanced CT scan shows large well-defined low-attenuation mass (M) with focal fat component (arrow) in right side of lower abdomen.

View larger version (93K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 10B. —Torsion of right ovarian cystic teratoma in 47-year-old woman with 7-day history of gradually increasing lower abdomen pain. Contrast-enhanced CT scan obtained caudad to A shows amorphous masslike structure (arrows) connecting mass and uterus (not shown); finding is suggestive of twisted vascular pedicle. Surgery revealed torsed right ovarian teratoma with hemorrhagic infarction.

Rupture
There is a low rate of spontaneous rupture of mature cystic teratomas (1.2–3.8%) [4]. Both acute and chronic clinical presentations are seen in intraperitoneal rupture. Acute peritonitis is caused by the sudden rupture of the tumor contents, usually in association with torsion, trauma, infection, or labor (Figs. 11A and 11B). Chronic granulomatous peritonitis results from a chronically leaking cystic teratoma and is the more common presentation.

View larger version (103K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 11A. —39-year-old woman with acute traumatic rupture of mature cystic teratoma. Contrast-enhanced CT scan shows large inhomogeneous low-attenuation right ovarian mass (R) with toothlike calcification in anterior wall at right side of abdomen; finding represents right ovarian cystic teratoma. Note also well-defined mass (L) with focal fat (arrow) in left side of abdomen; finding suggests left ovarian teratoma.

View larger version (108K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 11B. —39-year-old woman with acute traumatic rupture of mature cystic teratoma. Contrast-enhanced CT scan obtained cephalad to A shows focal disruption of medial wall of mass (arrowheads) with spillage of internal contents. Patient had acute lower abdomen pain after blunt abdominal trauma. Surgery confirmed ruptured right ovarian teratoma with hemorrhage.

On imaging, detection of discontinuity of the wall of the tumor is diagnostic for ruptured teratoma (Figs. 11A, 11B and 12A, 12B). Chronic granulomatous peritonitis shows the unusual imaging findings of ascites, hazy omental infiltration, and an inflammatory mass involving the omentum and bowel, mimicking those imaging findings of carcinomatous or tuberculous peritonitis [7] (Figs. 12A and 12B).

View larger version (98K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 12A. —75-year-old woman with chronic granulomatous peritonitis resulting from chronically leaking mature cystic teratoma. Contrast-enhanced CT scan shows ill-defined hazy infiltration in omentum and mesentery, mimicking peritoneal carcinomatosis.

View larger version (107K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 12B. —75-year-old woman with chronic granulomatous peritonitis resulting from chronically leaking mature cystic teratoma. Contrast-enhanced CT scan obtained caudad to A shows large cystic mass with eccentric dense calcification and fluid collection (F) in pelvic cavity. Note focal disruption of wall (arrow), suggesting rupture site of mature cystic teratoma. Surgery confirmed chronic granulomatous peritonitis resulting from rupture of mature cystic teratoma.

Malignancy Associated with Mature Cystic Teratomas
Malignancy associated with mature cystic teratoma is rare and complicates 1–2% of reported cases. It may occur either by malignant transformation of one of the preexisting benign elements or may represent a malignant lesion coexistent with a benign teratoma [5] (Fig. 13).

View larger version (119K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 13. —78-year-old woman with mature cystic teratoma engulfed by uterine endometrial stromal sarcoma. Contrast-enhanced CT scan shows large lobulated low-attenuation mass in midpelvic cavity, containing focal fat attenuation and rim calcifications (arrows) in posterior part. Surgery revealed large necrotic uterine sarcoma (high-grade endometrial stromal sarcoma) engulfing preexisting left ovarian mature cystic teratoma (arrows).

Malignant transformation usually occurs in postmenopausal women, in contrast to mature cystic teratoma, which is detected in women of reproductive age. In 85% of these cases, the malignant elements are composed of squamous cell carcinoma arising from the squamous lining of the cyst wall [5]. The Rokitansky protuberance is a common site of malignant transformation and should be sectioned appropriately during pathologic analysis.

On CT and MRI, ovarian teratoma with malignant transformation appears as a fat-containing tumor with an enhancing, irregularly marginated solid component (Fig. 14). The solid component tends to be relatively large and to show extensive transmural extension and direct invasion of neighboring pelvic organs. The contrast enhancement of the Rokitansky protuberance should raise the possibility of malignant transformation [8]. The imaging findings of malignant transformation may be similar to those of mixed germ cell tumors. Elevated serum -fetoprotein and human chorionic gonadotropin levels and younger age can help in the diagnosis of mixed germ cell tumors.

View larger version (131K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 14. —52-year-old woman with mature cystic teratoma with malignant transformation. Contrast-enhanced CT scan shows well-defined ovarian tumor with typical fat–fluid level, round mass of matted tuft of hair (arrowheads), and enhancing lobulated soft-tissue component (arrow) in anterior wall. Results of histopathologic examination confirmed squamous cell carcinoma arising in mature cystic teratoma.

Conclusion

Imaging findings of mature cystic teratomas can be atypical depending on the tumor components and the presence of combined complications. Understanding the atypical imaging manifestations of mature cystic teratomas permits a more specific and accurate diagnosis.

Acknowledgments

We thank Bonnie Hami, Department of Radiology, University Hospitals Health System, Cleveland, OH, for editorial assistance in preparing the manuscript.

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