Ovarian Germ Cell Tumors

Clinical Features

Dysgerminoma is one of the most common MGCTs of the ovary, but it accounts for only 1–2% of all ovarian cancers. They occur almost exclusively in children and young women. The average patient age is 22 years. Most patients present with a rapidly growing abdominal mass that often causes lower abdominal pain or pressure and may simulate pregnancy. Premenarchal patients with a pelvic mass should have their karyotype determined, as approximately 5% of dysgerminomas arise from gonadoblastomas in phenotypic females with abnormal gonads. These patients have pure gonadal dysgenesis (46,XY, bilateral streak gonads) or mixed gonadal dysgenesis (45,X/46,XY, unilateral streak gonad, contralateral testis). The estimated risk of malignancy is 28% by the age of 20 years for the former patients and 19% risk by the same age for the latter. Therefore, prophylactic removal of the gonads is recommended in both groups of patients at an early age. Almost all patients with dysgerminoma have elevated serum levels of lactic dehydrogenase (1 and 2 fractions) at presentation. In rare cases (3–5%), dysgerminomas may produce hCG and these patients may present with endocrine symptoms that are characteristically estrogenic (menstrual irregularities, isosexual pseudoprecocity, pseudopregnancy), but rarely androgenic. Serum levels of CA125, placental-like alkaline phosphatase (PLAP), and neuron-specific enolase (NSE) have been elevated in some cases. Paraneoplastic hypercalcemia has been described in some patients. One tumor occurred in a patient with a germline BRCA1 mutation. About two-thirds of dysgerminomas are stage IA at diagnosis; higher stage tumors involve the contralateral ovary (20%), pelvic and para-aortic lymph nodes, and/or the pelvic and abdominal peritoneum.

Macroscopic Features

Dysgerminomas are characteristically solid, and are well-encapsulated tumors with an average diameter of 15 cm. On section, they are lobulated, soft, and fleshy, and may appear gray-white or light tan ( Figure 29.3 ). Areas of coagulative necrosis and hemorrhage typically associated with cystic change may be seen. Such areas should be sampled to rule out the presence of other types of MGCT. The presence of calcification suggests an underlying gonadoblastoma. Dysgerminoma is the only MGCT that has a significant rate of bilaterality. Gross involvement of the contralateral ovary is seen in 10% of cases, and in another 10% microscopic foci of tumor are found.

Microscopic Features

The microscopic appearance of dysgerminoma is identical to that of testicular seminoma and extragonadal germinoma. It is composed of a monotonous population of rounded cells resembling primordial germ cells in a predominantly diffuse or insular arrangement. The cells may also grow as trabeculae, cords, or small nests ( Figure 29.4 ). Lack of intercellular cohesion may result in the formation of pseudoglandular spaces. The tumor cells are polygonal, with discrete cell membranes and abundant eosinophilic to clear, glycogen-rich, cytoplasm. The nuclei are large, central, and rounded, and they contain one or a few prominent nucleoli. Mitotic figures are often numerous. Aggregates of tumor cells are usually separated by thin fibrous septa almost always infiltrated by T-lymphocytes. In some tumors, lymphocytes are abundant and may form follicles with germinal centers. In approximately 20% of the cases, epithelioid sarcoid-like granulomas with multinucleated giant cells are present ( Figure 29.5 ). About 5% of dysgerminomas contain syncytiotrophoblastic giant cells (SGCs) in the absence of any other non-germinomatous differentiation. Such tumors, which have the same prognosis as dysgerminomas in which SGCs are absent, should be sampled extensively to rule out foci of choriocarcinoma or EC. The SGCs have a perivascular location, are immunoreactive for human chorionic gonadotropin (hCG)( Figure 29.6 ), and are often associated with stromal luteinization. The luteinized stromal cells, which may be admixed with the tumor cells or located at the periphery of the neoplasm, are probably responsible for the estrogenic or androgenic symptoms that are found in some patients.

Immunohistochemistry

Dysgerminoma cells show cytoplasmic and membranous immunoreaction for vimentin, PLAP, CD117 ( c-kit ), and D2-40 (podoplanin); the membranous reaction is characteristic of dysgerminoma. There is nuclear immunoreaction for the stem cell/primitive germ cell nuclear transcription factors OCT3/4, NANOG, and SALL4 ( Table 29.3 ). OCT3/4 is expressed very early during embryogenesis and has an essential role in blastocyst differentiation. However, when the female germ cells enter meiosis the expression of OCT3/4 is downregulated. OCT3/4 regularly shows positivity in dysgerminoma ( Figure 29.7 ) but can also be expressed in EC and in some immature neural elements of ovarian teratoma.

SALL4 can also show positivity in EC, YST, and primitive areas of immature teratoma (IT). Dysgerminomas may exhibit limited cytoplasmic dot- or rim-like staining for cytokeratin ( Figure 29.8 ) but epithelial membrane antigen (EMA) is negative. The SGCs that are found in a small percentage of dysgerminomas exhibit positive cytoplasmic staining for hCG ( Figure 29.6 ).

Somatic Genetics

Dysgerminomas, like seminomas, show by comparative genomic hybridization frequent (75%) chromosome 12p gain, usually in the form of an isochromosome 12p. The c-kit gene encodes a tyrosine kinase receptor (KIT), which is required in normal spermatogenesis and is expressed in seminomas and dysgerminomas. c-kit point mutations localized to exon 17, codon 816 (Asp>Val), involving the phosphotransferase domain, have been identified in 25–50% of ovarian dysgerminomas. The mutations are in exon 17, not in the exon 11 location, which confers susceptibility to imatinib therapy.

Genetic Susceptibility

Dysgerminoma is the most common malignant gonadal tumor in patients with gonadal dysgenesis and a Y chromosome. In these cases, the dysgerminoma typically arises in a gonadoblastoma.

Differential Diagnosis

Dysgerminoma should be distinguished from the solid type of YST, EC, clear cell carcinoma, and large cell lymphoma. The solid YST is almost always associated with other more typical patterns that are not seen in dysgerminomas. YSTs exhibit greater nuclear variation, contain hyaline bodies, immunoreact for AFP and glypican-3, and lack the lymphocytic infiltrate of dysgerminomas. Moreover, in contrast to dysgerminoma, YSTs fail to react for D2-40 and OCT3/4. The extremely rare EC shows larger cells with more ample nuclei that are more hyperchromatic and irregular than those of dysgerminoma. ECs are CD30 and cytokeratin positive, almost always contain SGCs, and lack the stromal infiltrate of lymphocytes. The distinction of dysgerminoma from clear cell carcinoma is discussed in Chapter 27 . Large cell lymphomas may resemble dysgerminomas both grossly and microscopically. Lymphomas, however, are bilateral in approximately one-half of the cases, and simultaneous involvement of the ipsilateral tube occurs in 25% of them (see Chapter 30 ). Careful attention to the nuclear features and immunostains for PLAP, D2-40, SALL4, and lymphoid markers facilitate the correct diagnosis. Poorly fixed dysgerminomas may occasionally be misdiagnosed as poorly differentiated carcinomas, as the cellular features of the former tumors are lost.

Treatment and Prognosis

The treatment of patients with dysgerminoma is primarily surgical, including the resection of the ovarian tumor and complete surgical staging. Given that the tumor mainly affects young females, for whom preservation of fertility is important, unilateral adnexectomy with frequent patient follow-up (every 2–3 months for the first 2–3 years) is the treatment of choice. This conservative approach is currently used in young patients even in the presence of metastasis. Therefore, biopsy of an apparently normal contralateral ovary is not recommended as it increases the risk of infertility. Dysgerminoma is highly sensitive to chemotherapy, which is usually reserved for the treatment of recurrent disease. Although these tumors are equally sensitive to radiotherapy, its use has been discontinued in an effort to preserve patient fertility, and cisplatin-based chemotherapy is currently preferred. In young patients whose karyotype analysis reveals a Y chromosome, both ovaries should be removed. When the patient's fertility is not a factor, hysterectomy with bilateral salpingo-oophorectomy is recommended; chemotherapy is subsequently administered in the case of higher stage tumors. Postoperative work-up includes the serum markers AFP and hCG, chest X-ray, and abdominopelvic CT or MRI. Recurrence in the retained contralateral ovary can occur in 5–15% of cases over the next 2 years. About 75% of the recurrences occur within the first postoperative year, and the most common sites are the peritoneal cavity and the retroperitoneal lymph nodes.

Patients with dysgerminoma have an excellent prognosis. The 5 year survival rate is greater than 95% for patients with stage IA tumor, and 85% for those with advanced stage or recurrent tumor. Features associated with recurrence include a tumor diameter greater than 10 cm, and age younger than 20 years. Although the designation ‘anaplastic dysgerminoma’ has been applied to tumors with numerous mitoses, there is no evidence that such tumors are associated with a worse prognosis, and therefore this term is not recommended. Most dysgerminomas are nondiploid and ploidy is not a useful prognostic factor.

Clinical Features

Yolk sac tumors (YSTs) (also referred to as primitive endodermal tumors ) account for approximately 10% of MGCTs and are almost as common as dysgerminoma in patients under the age of 20 years. They occur at a median age of 18 years and are rare over the age of 40 years. Patients most frequently present with abdominal pain and a large, rapidly growing, pelvic mass. Rupture or torsion of the tumor occurs in about 10% of patients. YSTs consistently produce AFP, which can be demonstrated in the patient's serum, usually at a level greater than 1000 ng/ml. This substance, which is normally produced in the yolk sac of the developing embryo, may serve as a tumor marker in monitoring the patient during and after therapy. Lower serum levels of AFP may be encountered in other tumors that also occur in young females such as IT and Sertoli–Leydig cell tumors (SLCTs). CA125 and carcinoembryonic antigen (CEA) are also elevated in 100% and 10% of YSTs, respectively. Rare examples of YST have occurred in older patients in association with mucinous or endometrioid tumors; in such cases, a somatic cell rather than a germ cell origin is almost certain.

Yolk sac tumor is a highly malignant neoplasm. At laparotomy, evidence of extraovarian spread has been reported in 30–70% of cases. In an older study, subclinical metastases were present in 84% of cases regarded as ‘stage I’ tumors, which most likely reflects incomplete staging. When the tumor spreads beyond the ovary, it involves the omentum and peritoneum, the para-aortic lymph nodes, and the liver.

Macroscopic Features

Yolk sac tumors are large, well-encapsulated masses, with an average diameter of 15 cm. The external surface is usually smooth and glistening. The sectioned surface is characteristically solid and cystic and shows soft, gray to yellow tissue with extensive areas of hemorrhage and necrosis ( Figure 29.9 ). When a polyvesicular vitelline component is present, the tumor shows a microcystic appearance. Although YSTs are bilateral in less than 5% of patients, the contralateral ovary may contain a dermoid cyst in about 10% of cases.

Microscopic Features

Although YSTs often exhibit a wide variety of microscopic patterns, most tumors have a reticular architecture reflecting extraembryonic differentiation. They are composed of a network of irregular spaces lined by primitive epithelial cells with glycogen-rich, clear cytoplasm, and large, hyperchromatic nuclei with prominent nucleoli; mitotic figures are numerous ( Figure 29.10 ). The most characteristic feature is the presence of isolated papillary projections with a central blood vessel and peripheral sleeve of embryonic epithelial cells (Schiller–Duval bodies; Figure 29.11 ). Cross sections of this structure once were erroneously compared with immature glomeruli (Schiller's mesonephroma). In fact, they closely resemble invaginations of yolk sac endoderm, as seen best in the rat placenta, forming the endodermal sinuses of Duval. Nevertheless, Schiller–Duval bodies are present in only 20% of cases. The term ‘endodermal sinus tumor’ is misleading, since the endodermal sinus is neither a structure present in human embryogenesis nor a constant feature of these neoplasms. Accordingly, the designation primitive endodermal tumor has recently been proposed. Another distinctive feature is the presence of brightly eosinophilic, periodic acid–Schiff (PAS)-positive, diastase-resistant hyaline globules ( Figure 29.10 ). Occasionally, multiple small vesicles with eccentric constrictions resembling the yolk sac vesicle of the normal embryo are present, and the tumor is designated a polyvesicular vitelline tumor ( Figure 29.12 ). This pattern recapitulates the subdivision of the primary yolk sac vesicle into a large component lined by flat cells (vestigial primary yolk sac) and a smaller component lined by taller epithelium that simulates the forerunner of the primitive gut and its appendages (secondary yolk sac vesicle) in normal embryogenesis ( Figure 29.13 ). Like the normal yolk sac vesicle, the neoplastic yolk sac may give rise to tumors of embryonal type. These tumors recapitulate primitive gut (glandular YST) and primitive liver (hepatoid YST).

Endodermal-type glands may be found in approximately 50% of YSTs, often admixed with reticular and polyvesicular vitelline components. In some tumors, a glandular pattern predominates, and may appear as rounded cribriform aggregates of primitive epithelial cells (‘intestinal’ variant; Figure 29.14 ), or glands of ‘endometrioid’ type, which can be mistaken for typical or secretory endometrioid adenocarcinoma (endometrioid-like variant; Figure 29.15 ); foci of carcinoid tumor have been described in one of the latter tumors. Occasionally, the mature intestinal component may give rise to a mucinous carcinoid.

Although small foci of hepatoid differentiation are found in 16–48% of YSTs, neoplasms with a predominant hepatoid component are infrequent. These tumors are characterized by compact masses of large polyhedral cells with abundant eosinophilic cytoplasm, and round central nuclei with prominent single nucleoli, separated by thin fibrous bands, resembling hepatocellular carcinoma ( Figure 29.16 ). Hyaline bodies are usually numerous. In some cases, glandular spaces containing mucin are present. Ultrastructural examination of hepatoid YSTs reveals features similar to hepatocellular carcinoma. A ‘parietal’ differentiation characterized by small extracellular accumulations of basement membrane material has been described in over 90% of YSTs. Rarely, YSTs may contain SGCs.

The loose stromal component of YST recapitulates the extraembryonic mesenchyme (magma reticulare), and may be the site of origin of the sarcomas that develop in some patients after chemotherapy.

Immunohistochemistry

Similar to the primitive gut (secondary yolk sac) and its appendages, YSTs are immunoreactive for AFP ( Figure 29.17 ), glypican-3 ( Figure 29.18 ), SALL4 ( Figure 29.19 ), villin, CDX2. and hepatocyte paraffin antigen 1. AFP is expressed often focally either as granular cytoplasmic deposits or delineating intracellular canaliculi. Glypican-3 immuno­reaction is usually stronger than that of AFP; however, it is not as specific since it is expressed by other germ cell tumors and clear cell carcinoma. Nuclear transcription factor SALL4 is regularly expressed in both epithelium and mesenchyme and there is cytoplasmic reactivity of RNA-binding protein LIN28. Villin is also constantly expressed in membranes and cytoplasms of epithelial cells. The different endodermal elements may be immunoreactive for their corresponding tissue markers, i.e., hepatic components for hepatocyte paraffin antigen 1, intestinal elements for CDX2, and foregut-derived epithelia for thyroid transcription factor 1. However, YSTs are negative for OCT4, SOX2, D2-40, and CD30 ( Table 29.3 ). Combined expression of AFP, glypican-3, villin, SALL4, or LIN28 makes an ideal panel for confirming the diagnosis.

Differential Diagnosis

Because of their various microscopic patterns, YSTs can be confused with many other ovarian tumors, both primary and metastatic. The most common problems in differential diagnosis include clear cell carcinoma and endometrioid adenocarcinoma (see Chapter 27 ), the retiform variant of SLCT (see Chapter 28 ), and hepatoid carcinoma (see Chapter 27 ). YSTs should also be distinguished from other germ cell tumors such as dysgerminoma and the very rare EC. These differential diagnoses are discussed under those headings.

Treatment and Prognosis

The treatment of YST includes surgical exploration, unilateral salpingo-oophorectomy, and frozen section for diagnosis. Although gross metastases should be removed, a thorough surgical staging is not necessary because all patients need chemotherapy. Prior to combination chemotherapy, the prognosis for patients with YSTs was dismal despite apparently adequate surgery. In one large study of patients treated prior to 1975 the 3 year survival was only 13%. The development of the vincristine, dactinomycin, and cyclophosphamide (VAC) combination chemotherapy dramatically changed the outlook for patients with YSTs. Currently, cisplatin-based chemotherapy provides still better results. Survival rates have approached 100% for patients with stage I tumors, and 75% for patients with higher stage tumors. Serum AFP can be used to monitor the response to treatment and to detect tumor recurrence. Adverse prognostic factors include tumor stage II or higher, gross residual tumor after surgery, and ascitic fluid of more than 100 ml. The good long-term outcome in two polyvesicular-vitelline tumor (PVVT) cases supports prior evidence that this form of YST may be more indolent than the conventional forms of the tumor.