In mixed malignant germ tumors, the most malignant type defines the prognosis. Serum levels of HCG and AFP may assist in the diagnosis and in the follow-up of some germ cell tumors.

Malignant germ cell tumors comprise, in order of decreasing frequency, dysgerminomas, immature teratomas, endodermal sinus tumors, and embryonal and nongestational choriocarcinomas. The latter three are extremely rare. In these patients, tumor markers may be helpful for assessing response and tumor recurrence. Endodermal sinus tumors secrete AFP (Fig. 22). Embryonal carcinomas can secrete both AFP and HCG, whereas pure choriocarcinomas secrete only HCG (Ozols et al. 2001).

Dysgerminomas

Dysgerminomas present the most common type of malignant germ cell tumors and have been considered the female counterpart of seminoma of the testis. Seventy-five percent occur in early reproductive age, 10% in prepubertal girls, and 15–20% are diagnosed during pregnancy or postpartally (Hricak et al. 2004). In contrast to the other germ cell tumors, dysgerminomas may also occur bilaterally.

A minority (5%) of dysgerminomas arise in gonadal dysgenesis and may coexist with gonadoblastomas. Disorders of sexual differentiation, e.g., Turner syndrome and Swyer syndrome, harbor a considerably increased risk of dysgerminomas arising in streak gonads during adulthood (Tayfur et al. 2007). The vast majority of patients with dysgerminomas are diagnosed with early-­stage disease, and fertility saving is an option in stage IA disease.

Imaging Findings

Dysgerminoma presents as a multilobulated, welldelineated solid lesion. In CT, speckled calcifications may be observed. Furthermore, they may contain low attenuation areas representing necrosis or hemorrhage. Contrast-enhanced CT may also demonstrate strongly enhancing fibrovascular septa. In MRI, the tumor displays low signal intensity on T1-weighted images and intermediate signal with low SI septa and high signal intensity areas of necrosis on T2-weighted images and restricted DWI (Fig. 23) As in CT, the intralesional septa may display strong enhancement (Tanaka et al. 1994).

Differential Diagnosis

Differential diagnosis includes solid ovarian tumors in younger age, e.g., granulosa cell tumors, yolk sac tumors, Sertoli cell tumor, and immature teratomas. The extremely rare ovarian lymphoma typically involves both ovaries. In MRI, uterine fibroma and fibrothecoma may display a similar appearance on T2-weighted images; however, contrast enhancement of uterine leiomyomas resembles that of myometrium, and fibrothecoma displays a type 1 time-intensity curve on DCE MRI (Forstner et al. 2016a). In CT, differentiation of subserosal uterine fibroids or ovarian fibromas from solid dysgerminomas is usually not feasible.

Immature Teratomas

Immature teratomas or malignant teratomas are the second most common germ cell malignancies. The typical age group is similar to dermoid cysts young age between 10 and 20 years. However, in contrast to benign teratomas, they are extremely rare, with less than 1% consisting of immature teratomas. They are typically large at the time of diagnosis and present as solid or predominantly solid tumors with cystic elements and areas of fat and calcifications. Immature teratomas are associated with dermoid cysts, more commonly in the ipsilateral (26%) than in the contralateral ovary (Young and Scully 2002a; Heifetz et al. 1998). They contain embryonic tissues and can also occur in combination with other germ cell tumors (mixed germ cell tumors). Yolk sac tumors within immature teratomas give rise to alpha-fetopro- tein elevation and are an important prognostic factor (Heifetz et al. 1998). Immature teratomas may also rarely produce steroids and cause pseudoprecosity in prepubertal girls (Young and Scully 2002a; Heifetz et al. 1998).

Imaging Findings

Immature teratomas typically occur in young females and display mostly as heterogenous, predominantly solid lesions or as mixed solid and cystic lesions with scattered or coarse ­calcifications or hemorrhage (Heifetz et al.