Primitive neuroectodermal tumour of the CNS

Primitive neuroectodermal tumours (or CNS PNET) are agressive neoplasms of the brain, most frequently encountered in the paediatric population. 

A number of tumours fall under the umbrella term CNS PNET ⁷:

• medulloblastoma : by far the most common
• supratentorial PNET : 15% of all CNS PNET
  • pineoblastoma : most common supratentorial PNET
• spinal PNET : rare

The article focuses on the supratentorial primitive neuroectodermal tumour (sPNET) which account for the minority of cases. Medulloblastomas and pineoblastomas are discussed separately. 

CNS PNETs should not be confused with peripheral primitive neuroectodermal tumour (pPNET) which are musculoskeletal tumours. 

Epidemiology

They typically occur in the paediatric population, usually during the first 10 years of life, with a mean age of 5 years ⁷. There is a recognised male predilection.

Clinical presentation

Clinical presentation is non-specific and cannot be distinguished from other agressive intracranial masses. Symptoms of raised intracranial pressure, seizures and focal neurology may be present. 

Pathology

Supratentorial PNETs are composed of primitive undifferentiated neuroepithelial cells, and are WHO grade IV tumours. They are small round blue cell, malignant embryonal tumours which can show divergent differentiation of variable degree along neuronal, glial, or rarely mesenchymal lines. They are extremely variegated in histology and this reflects in their imaging characteristics. Homer Wright rosettes are seen.

Associations

• Gorlin-Goltz syndrome
• Turcot syndrome

Radiographic features

Supratentorial PNETs, by definition, occur above the tentorium, and can either arise in the cerebral hemispheres, or within the lateral ventricles. 

CT

• often seen as a large irregular mass
• typically iso to hyper-attenuating on non contrast imaging ⁷
• cystic components are common ( ≈ 65% ⁶)
• calcification can be common ( ≈ 70 % ⁶)
• shows heterogenous contrast enhancement

Angiography

Non specific and range from avascular to hypervascular ⁶.

MRI

The extent of peritumoural vasogenic oedema is surprisingly low given the size of the mass and the agressive tumour characteristics ⁷.

• T1 : highly variable and can be hypointense to isointense
• T2 :
  • generally high signal solid components
  • cystic components are common
  • low signal portions due to calcific compoments
• T1 C+ (Gd) :
  • shows markedly heterogenous enhancement 
  • leptomeningeal seeding is common
• DWI : often shows rescricted diffusion
• MR spectroscopy : choline levels are decreased resulting in  NAA / Cho and Cr / Cho ratios lower than astrocytomas ⁷

Treatment and prognosis

Supratentorial PNET's generally have a poorer prognosis than medulloblastomas, with a low 5 year survival rate. Leptomeningeal and / or subarachnoid seeding are common, necessitating imaging and treatment of the whole neuraxis. 

Differential diagnosis

General imaging differential considerations include

• astrocytoma
• ependymoma
• oligodendroglioma
• intracranial teratoma
• atypical teratoid / rhabdoid tumour (AT / RT)