Ovarian germ cell tumours make up 30% of ovarian neoplasms; nearly all of these are mature cystic teratomas (dermoid cysts) with only 5-10% of tumours comprising malignant histologies. There is a similar spectrum of germ cell histologies to male germ cell tumours, albeit with different names in several cases.
Epidemiology
These tumours are the most common type to occur in children, adolescents and young adults, making up 60% of diagnoses. One third of these will be primitive types, and the remainder mature teratoma. Primitive germ cell tumours are rare in older women.
Aetiology
Unknown. The cells are thought to arise from primordial germ cells rather than from the mature oocytes of the ovary.
Clinical Features
Some tumours (eg. nongestational choriocarcinoma and embryonal carcinoma) may produce extremely high levels of β-hCG, which can have hormonal effects on the patient (e.g. precocious puberty, amenorrhoea). Most other patients present with a mass and pain.
Examination can demonstrate extremely large abdominal tumours.
Pathological Features
Primitive Germ Cell Tumours
Dysgerminoma
Dysgerminoma is the female equivalent of seminoma in men. Macroscopically, the tumour is pale, lobulated and with a circumscribed border. Microscopically, the malignant cells are arranged in lobules surrounded by a fibrous septa with lymphocytic infiltrate. The cells themselves have distinct cell borders, pale cytoplasms and polygonal shape with a regular, small nucleus. Immunohistochemistry is positive for placental alkaline phosphatase (PAP or PLAP).
Yolk Sac Tumour
This is a poorly named tumour as there are multiple appearances other than 'yolk sac'. They form structures associated with the endoderm, either extraembryonal or somatic. The tumour is macroscopically grey to yellow, with substantial areas of necrosis and haemorrhage. Microscopically, there are several appearances:
- Extraembryonal types
- Primitive endoderm/yolk sac: There are a variety of appearances; the reticular pattern is formed by a mesh of myxoid stroma lined by atypical epithelial cells. The solid pattern consists of solid growth of epithelioid cells with clear cytoplasm. Endodermal sinus pattern is uncommon and consists of cells arranged around a central lumen.
- Other subtypes include allantois (polyvesicular pattern) and murine-like yolk sac (parietal pattern).
- Somatic types may give rise to primitive lung, gut or liver appearances (glandular, hepatic patterns)
More importantly, these cells stain positively for alpha fetoprotein (AFP) which differentiates them from dysgerminoma.
Embryonal Carcinoma
Embryonal carcinoma is a very rare ovarian malignancy that recapitulates the early stages of embryonal development. Microscopically, tumour cells form vague gland-like structures. The cells are large and pleomorphic. There are often interspersed syncytiotrophoblasts which are β-hCG positive.
Polyembryoma
Considered a subtype of embryonal carcinoma, this tumour forms multiple blastocyst-like structures with embryonal discs.
Nongestational Choriocarcinoma
Much rarer than gestational trophoblastic disease, nongestational choriocarcinoma is often large, haemorrhagic and necrotic macroscopically. Microscopically the tumour is formed by syncytiotrophoblast (multinucleated cels) and cytotrophoblast (single nuclear cells) often arranged in pseudopapillae. Immunohistochemistry is positive for β-hCG. More than any other type, these tumours can cause precocious puberty/amenorrhoea.
Mixed Germ Cell Tumours
The above patterns, with the exception of dysgerminoma, are most commonly seen mixed with each other or with a mature/immature teratoma. Serum markers (AFP/β-hCG) give clues as to the likely components. Dysgerminoma is often seen as a component. Importantly, metastatic disease may arise from a single part of the mixed tumour, or each part may metastasise separately.
Teratoma
Teratomas consist of components arising from one, two or three germ layers (endoderm, mesoderm, ectoderm).
Immature Teratoma
These form about 1% of all ovarian malignancies and 20% of malignant germ cell tumours; they are most common during the first twenty years. Macroscopically they can be very large, grey, haemorrhagic and necrotic. The most common form contains neuroectodermal tissue (small round blue cells) arranged in rosettes with a high mitotic rate. Other immature or mature types may be present, including mesodermal structures (primordial fat, muscle cells) or endodermal structures (primordial renal, hepatic or gut cells). It is curable in many cases with surgery, but chemotherapy raises the cure rate to close to 100%.
Mature Teratoma and Dermoid Cyst
This tumour contains only mature, adult tissue from the germ layers. They are the most common germ cell tumour (about 30% of all ovarian neoplasms) and are benign. They are most often cystic (dermoid cyst) but may be solid. They often contain structures such as hair, skin, or teeth; rare forms may contain structures resembling a malformed fetus. The dermoid cyst is a large cyst (often > 10 cm) that is filled with hair and sebaceous fluid (ick).
Monodermal Teratomas
These are teratomas that contain only one germ layer. They often give rise to other malignancies (e.g. glioblastoma, melanoma, carcinoid tumour).
