Prepared by Dr. D.R. Petty, Radiology Registrar, St. James' University Hospital; and Dr. J.A. Spencer, Consultant Radiologist, St. James' University Hospital, Leeds, United Kingdom.

CASE SUMMARY

A 28-year-old woman presented to her primary care physician with an 8-year history of primary subfertility. Pelvic ultrasound performed as part of the investigation of this condition demonstrated a left ovarian mass (Figure 1). A pelvic magnetic resonance (MR) scan was subsequently performed to further characterize this lesion (Figures 2 and 3). A left salpingo-oophrectomy was performed.

What was the diagnosis?

IMAGING FINDINGS

The transvaginal ultrasound demonstrated a 4.4 * 6.0 cm cystic mass containing a solid component, some of which was hyperechoic, suggesting the presence of fat. One area demonstrated acoustic shadowing, in keeping with a focus of fat or calcification.

The pelvic MR demonstrated a complex 8.0 * 5.0 * 5.0 cm left ovarian mass (Figure 2), parts of which demonstrated significant T1 signal loss with fat suppression (Figure 3). In addition, there was a small focus that returned low signal on all sequences. These characteristics are in keeping with mature fat and calcification, respectively.

PATHOLOGY

The specimen consisted of a soft, smooth-surfaced mass attached to the left fallopian tube. Pathologic examination revealed that it was a cystic mass containing hair, sebaceous material, fat, and bone.

DIAGNOSIS

Left ovarian mature teratoma

DISCUSSION

Mature cystic teratomas (MCT), also known as dermoid cysts, are the most common form of ovarian teratoma (which also includes monodermal teratomas, such as struma ovarii and immature teratomas). ¹ These entities contain well-differentiated cell lines from at least two of the germ-cell layers. ² They account for 25% of all ovarian tumors and are bilateral in 10% to 25% of cases. ³ Ovarian MCT affect a younger age group than ovarian epithelial tumors with a mean age of 30. They are usually asymptomatic, ⁴ although complications, such as torsion, can occur. Malignant degeneration occurs in approximately 2% of cases, usually in postmenopausal women. ¹ Occasionally, the cyst can rupture, which produces granulomatous peritonitis; this is said to occur in <1% of cases. ⁴

The diagnosis of ovarian MCT can sometimes be made on a plain abdominal radiograph if a radiolucent pelvic mass containing a tooth is demonstrated.

Ultrasound appearances are often characteristic due to the presence of a highly reflective dermoid plug (Rokitansky nodule) that is the solid element within the cyst. This contains hair follicles, sebaceous glands, fat, and calcified or ossified elements. It usually forms an acute angle with the wall of the cyst, ⁵ and can produce acoustic shadowing due to the presence of hair, calcium, or bone. A fluid-fluid level may be detected due to sebum floating on an aqueous, more echogenic, layer. ² Echogenic lines and dots may be seen within the fluid, caused by strands of hair in the cyst, although fibrinous strands in hemorrhagic cysts ⁶ can also cause this appearance.

Tubo-ovarian abscesses can also contain fluid-fluid levels and echogenic pus and produce acoustic shadowing due to gas. Ectopic pregnancies also demonstrate shadowing from bones and contain echogenic hemorrhage, which may separate to give fluid-fluid levels. Therefore, these conditions can mimic ovarian MCT sonographically, although the clinical setting should allow accurate diagnosis in the majority of these cases. ⁶ Hemorrhagic cysts can also cause diagnostic difficulty; however, the echogenic focus produced by fresh hemorrhage displays through transmission rather than acoustic shadowing. ⁷

Ovarian MCTs are the most commonly missed ovarian neoplasm on sonography, often due to the tip of the iceberg sign, in which the back wall of the cyst is obscured by acoustic shadowing, causing the echogenic Rokitansky plug to be misinterpreted as bowel gas. ⁷

Computed tomography (CT) and MR are more sensitive to the presence of fat and calcium than is ultrasound, thus making the diagnosis using these modalities more straightforward. ² On CT, the demon-stration of fat within an adenexal mass is highly suggestive of an ovarian MCT. ⁸ It was present in 93% of cases in one series, with teeth or other calcifications present in 56%. ⁹ Contrast enhancement is unusual in benign MCT and is suggestive of malignant change. ⁵ The presence of malignancy should also be considered if the solid element within the cyst measures >5 cm.

With MR, the intracystic sebum and the adipose tissue usually present in the Rokitansky nodule typically have signal characteristics of high signal on T1-weighted images and intermediate signal on T2-weighted images. These findings are not diagnostic of ovarian MCT, as they can also occur in intracyst hemorrhage, eg, in endometriomas. Hemorrhagic products in an endo-metrioma can also produce a layered appearance on T2-weighted images or sometimes a gradated signal loss called shading, a finding that is not seen in MCT.

The loss of high signal on T1-weighted images with fat suppression is diagnostic of MCT, allowing it to be differentiated from hemorrhagic lesions. Occasionally nondependant spheres of intracyst lipid are demonstrated within the MCT. ⁵ Areas of calcification and ossification are of low signal on all sequences on MR. A salt-and-pepper speckled appearance can sometimes be seen within an MCT, which is thought to be due to multiple chemical shift artifacts at fat/water interfaces.

The diagnosis of ovarian MCT can often be made confidently on ultrasound; in one study, experienced observers were able to achieve 100% positive predictive value in cases in which two or more characteristic sonographic features were present. ⁶ In uncertain cases, MR and CT will increase diagnostic accuracy due to their high sensitivity for fat and calcium content. MR is the more sensitive of the two modalities, ¹⁰ and has the added advantage of not involving ionizing radiation in the young-adult age group.

Due to the potential for complications, such as torsion and rupture, MCTs are usually resected. Therefore, gynecological referral is recommended upon their diagnosis.