Fig. 23  Dysgerminoma of the right ovary in a 32-year-­old female. A large, well-delineated multinodular solid lesion is located cranially and anterior of the uterus (a). It displays predominantly intermediate SI on the T2-weighted

image (a) and moderate contrast enhancement (b), but clearly restricted diffusion (b = 1,000 mm2) on DWI (c). Multiple septa (arrows) can be identified

1998; Bazot et al. 1999; Yamaoka et al. 2003). In CT, punctate foci of fat and calcifications are diagnostic clues for the presence of an immature teratoma (Bazot et al. 1999). In case of cystic lesions, they are typically filled with serous fluid and may rarely contain fatty sebaceous material (Diop et al. 2014). In MRI, small foci of fat with high SI on T1 (Fig. 24) and signal loss on the fat saturation sequence are typically found (Yamaoka et al. 2003). Malignant type of enhancement of the solid aspects may be seen. Capsular penetration is found in almost 50% of cases and is a pathognomonical feature of malignancy (Comerci et al. 1994).

Malignant Transformation in Benign Teratoma

Malignant transformation of benign teratomas is rare and reported in 0.17% of dermoid cysts (Comerci et al. 1994). It is associated with advanced age (mean 59 years) and a large (>6 cm) unilateral benign teratoma. In the vast majority, squamous cell cancer (up to 85%), or rarely carcinoid tumors, and adenoor chorionic cancer arise from the cyst wall or from ectodermal elements of benign teratomas (Choudhary et al. 2009).

Imaging Findings

Fat within an ovarian mass is diagnostic of a teratoma. Signs indicative of malignancy include a

a

b

Fig. 24  Mature and immature teratoma in a 20-year-old female. T1weighted image (a) and T2-weighted image with FS (b) at the acetabular level. Ascites surrounds bilateral ovarian lesions. The left tumor (*) represents a benign dermoid with predominantly fatty tissue. Posteriorly an inhomogeneous mixed solid and cystic lesion (arrow) with small hemorrhagic loculi is seen, which is better identified on the T2-weighted image (b). The tiny spots of high SI on T1-weighted image represent areas of fat (arrow) in (a). Courtesy of TM Cunha, Lisbon

a

solid well-vascularized large mural nodule, often arising from the Rokitansky protuberance, breach of the capsule, or extracapsular growth and metastases (Fig. 25) (Choudhary et al. 2009; Kido et al. 1999). Elevation of tumor markers CEA and CA-125 in an older female with a large fat-/sebaceum-containing mass is diagnostic of malignant degeneration of a dermoid cyst (Dos Santos et al. 2007).

Differential Diagnosis

Immature teratomas are usually large at presentation and occur in young females. In contrast to the majority of benign cystic teratomas, malignant teratomas tend to be predominantly solid with small foci of fat and scattered calcifications. Elevation of alpha-1-fetoprotein assists in establishing the diagnosis and is found in 33–65% of immature teratomas (Yamaoka et al. 2003). Mature and immature teratomas coexist in approximately 20% of cases. If no fat is identified, an immature teratoma cannot be differentiated from a monodermal benign teratoma, e.g., of struma ovarii, from malignant germ cell tumors, or from ovarian cancer (Dujardin et al. 2014). Pitfalls include struma ovarii, where nodules may show avid contrast enhancement similar to malignant degeneration (Forstner et al. 2016a). However, only capsular breach proves the malignant transformation (Choudhary et al. 2009).

b

Fig. 25  Malignant transformation. In a 64-year-old female, a mural nodule breaching the capsule of a benign teratoma is seen on the T2WI (arrow) (a). FS GdT1 WI

(b) shows the intensely enhancing nodule (arrow) with extracapsular growth. Fat (*) within the teratoma. U uterus