Central nervous system germinoma

Intracranial germinomas (also known as dysgerminomas, or extra-gonadal seminomas) are a type of germ cell tumour, and are predominantly seen in paediatric populations. They tend to occur in the midline, either at the pineal region (majority) or along the floor of the third ventricle / suprasellar region. 

Epidemiology

Germinomas are tumours of young patients with a peak incidence of 10 - 12 years of age (90% of patients being younger than 20 at the time of diagnosis). They account for 3 - 5 % of paediatric intracranial tumours but only 0.4 - 1% of intracranial tumours in adults ¹⁰ .

They are the most common tumour of the pineal region accounting for approximately 50% of all tumours, and the majority (73 - 86%) ¹⁰ of intracranial germ cell tumour. 

Gender ratios are interestingly different depending on location. In the pineal region there is a marked male preponderance with a male to female ratio of 5 - 22:1 whereas in the suprasellar region, they are slightly more frequent in females with a male to female ratio of 1:1.3 . Overall, due to the pineal region being most common, germinomas are seen more frequently in males.

Clinical presentation

Presentation depends on location, with compression of the tectal plate leading to obstructive hydrocephalus and Parinaud syndrome, whereas involvement on the pituitary infundibulum leads to diabetes insipidus (most common), hypopituitarism (common) or optic chiasm compression / sings of intracranial hypertension ¹¹. When the thalami and basal ganglia are involved, then presentation is often delayed with a larger tumour at diagnosis.

Pathology

Germinomas derive from germ-cells and typically arises in the the midline, relating to the third ventricle ¹¹:

• pineal gland : 80 - 90% : known as a pineal germinoma in this location
• suprasellar mass : 15-40% : termed a suprasellar germinoma in this location
• paraventricular parts of the basal ganglia and thalamus : 5 - 10%
  • more frequently seen in Asian populations
• both pineal and suprasellar region: 6-13%¹¹

Radiographic features

Germinomas are soft tissue density, enhancing masses. When present in the pineal region they appear to "engulf" normal pineal tissue and can have associated central calcification, in contrast to pineocytomas, and pineoblastomas which are described as "exploding" the foci of calcification. Cystic components are common, found in upto 45% of cases.

CT

On CT the high cellularity results in a degree to hyperdensity compared to adjacent brain. Usually the mass enhances brightly.

When in the floor of the third ventricle it is typically seen filling and expanding the infundibular recess and supraoptic recess. Imaging may however be normal initially and if the diagnosis is suspected clinically (e.g. idiopathic hypothalamic diabetes insipidus) then close followup is required to identify potentially very suble abnormal pituitary stalk ehnancement and thickening.

In the paediatric population presence of calcification in the pineal region is a useful marker of an underlying tumour, as no calcification of the pineal is seen in children below the age of 6.5 and in only ~10% of children between 11 and 14 years of age.

MRI

MRI demonstrates a soft tissue mass, typically ovoid or lobulated in contour, with the following signal characteristics ¹²: 

• T1
  • isointense or slightly hypointense to adjacent brain
• T2
  • isointense or slightly hyperintense to adjacent brain
  • may have areas of cyst formation 
  • may have areas of haemorrhage (low signal)
  • have a predilection to invade adjacent brain (oedema)
• T1 C+
  • vivid and homogeneous

Treatment and prognosis

The mainstay of treatment is with radiotherapy which offers a 79 - 90% long term cure rate ¹⁰. Leptomeningeal seeding or spillage during surgery are thought to be poor prognostic factors. Imaging of the entire neuraxis is therefore recommended prior to irradiation.

Overall the prognosis is good, with over 90% 5 year survival with chemotherapy and radiotherapy.

Differential diagnosis

The differential can depends on location:

• for lesions in the pineal region : refer to pineal mass article
• for lesions in the suprasellar region : refer to suprasellar mass article

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