Mediastinal teratoma

Mediastinal teratomas are germ cell tumours located in the anterior mediastinum.

Epidemiology

Mediastinal teratomas are, representing the most common extra-gonadal germ cell tumours.

Epidemiology

They account for approximately 15% of anterior mediastinal masses in adults and approximately 25% of anterior mediastinal masses in children. They are by far the most common mediastinal germ cell tumour, accounting for 50-70% of such tumours ⁹.

The age of presentation is wide, but the typical presentation in adults is in the third or fourth decade (20s(the 20s and 30s). In children below 1one year of age, immature teratomas are common (40%) and may be detected antenatally ⁹.

No definiteparticular gender predilection for mature teratomas has been identified (at most there is a slight female predilection). Immature teratomas, however, occur almost exclusively in males ⁹. 

Clinical presentation

The majority of patients are asymptomatic, with the mediastinal mass discovered incidentally when the thorax is imaged for another reason ^7,9. Masses that become symptomatic can do so in a variety of ways:

• mass effect
  • respiratory distress (infants)
  • respiratory failure
  • superior mediastinal syndrome
  • neck mass
  • Horner syndrome ⁹
• endocrine function
  • hormone production, e.g. beta-HCG, insulin
• rupture
  • chest pain
  • haemoptysis
  • respiratory distress
  • cardiac tamponade ⁹
  • pleural effusions ⁹

Pathology

Mediastinal teratomas are germ cell tumours arising from ectopic pluripotent stem cells that failed to migrate from yolk endoderm to the gonad. By definition, they should contain elements from all three embryological layers: endoderm, mesoderm and ectoderm. Frequently however elements from only two layers are evident ⁷ (see teratoma article).

Amediastinal dermoid cyst can be considered a variant of a mature teratoma, predominantly formed by squamous epithelium and skin appendages (ectoderm and mesoderm respectively) ⁹.

Teratomas may either be:

• mature: well differentiated
• immature: poorly differentiated
• with malignant transformation
  • mature teratoma with non-germ cell malignancy arising from one of the components ²

Because they ariseoriginate from primitive cells, they have variable neoplastic potential. Generally, cystic lesions tend to be benign whereas solid lesions tend to be malignant; however the final diagnosis is made histologically.

Mature teratomas and most immature teratomas are benign tumours, but still, carry a risk of malignancy despite being indolent initially and require close clinical, serological, and radiological follow-up, or surgical excision ^4-5,9.  There is also a low incidence of malignant transformation of somatic cells (i.e. non germ-germ cell components) within these tumours, e.g. carcinoma, sarcoma, leukaemia ⁶.

Location

Usually occurs within or near the thymus gland.

Associations

Mature teratomas have been associated with:

• Klinefelter syndrome (47, XXY) ⁹

Immature teratomas can be associated with:

• non-lymphocytic leukaemia
• pleomorphic undifferentiated sarcoma

Radiographic features

The vast majority of mediastinal teratomas are located in the anterior mediastinum (80%), with most of the remainder involving multiple compartments (13-15%). Isolated posterior or middle mediastinal location is uncommon (2-8%) ⁹.

Mature teratoma

Mature teratomas, by their very nature, may contain a very widevast range of tissues and thus have variable imaging features. In general, they have the following features:

• well-demarcated, displacing rather than invading adjacent structures
• large: 3-25cm ⁹
• usually cystic: 90% ⁹
  • may be uni- or multiloculated
• septal/rim contrast enhancement
• variable attenuation, consistent with different tissues
  • fat
  • water density cystic spaces
  • fat-fluid levels (specific)
  • homogeneous soft-tissue density
  • calcification: 26% ⁹
    • identifiable teeth or bone is seen in up to 8% of cases ⁹

When rupture has occurred, the density of each component is less likely to be homogeneous, but rather each part of the teratoma is heterogeneous (i.e. the fat component is heterogeneous, as is the cystic component etc.) ⁷.

Immature

Immature teratomas are usually solid.

Plain radiograph

Appearances on chest radiography are usually indistinguishable from many of the other causes of an anterior mediastinal mass. Calcification may be visible.

CT

CT is the mainstay of diagnosis. The appearance will depend on the type of teratoma, and whether or not a cystic teratoma has ruptured. They are all however usually located in the anterior mediastinum.

Treatment and prognosis

Treatment depends on whether the teratoma is mature or immature. In the former, surgical resection is curative. In the later management depends on alpha-FP levels. If these are elevated then postoperative chemotherapy is usually employed ⁸. 

Prognosis of mature teratomas is excellent as they are benign tumours. Pure immature teratomas also usually have an excellent prognosis, especially in childhood. In up to 30% of cases, however, immature teratomas have a malignant germ cell tumour component (most frequently yolk sac tumour), which result in frequent (25%) recurrence. Nonetheless, with adjuvant chemotherapy a greater than 80% 3 yearthree-year survival can be achieved in children. Prognosis in adults is less favourable ⁹.

Potential complications include

• respiratory distress/failure: infants>adults
• haemorrhage
• pneumothorax
• fistula(e)  formation: aorta, SVC, oesophagus, bronchus
• rupture
  • bronchus: trichoptysis is pathognomic
  • pericardium: pericardial effusion and tamponade
  • pleural cavity: pleural effusion

Differential diagnosis

On chest radiography, the differential is broad, and includes any cause of an anterior mediastinal mass: 

• differential diagnosis for anterior mediastinal tumours
• mnemonic: 5 T's.

On CT the differential inis usually narrowed to tumours, and depends on the morphology.:

• mass containing fat
  • thymolipoma
• heterogeneous soft tissue
  • other germ cell tumours: mnemonic SECTE
  • lymphoma
  • thymic tumours
• predominantly cystic
  • thymic cyst
  • bronchogenic cyst