Primitive neuroectodermal tumour of the CNS

Dr Jeremy Jones et al.

Primitive neuroectodermal tumours (or CNS PNET) were considered aggressive neoplasms of the brain, most frequently encountered in the paediatric population. Importantly, the term no longer appears in the current WHO classification of CNS tumours 11-12

The term has fallen out of favour and has been removed from the 2016 update to the WHO classification of CNS tumours, now incorporated into the term embryonal tumors with multilayered rosettes (ETMR) 10

Traditionally a number of tumours variably fell under the umbrella term CNS PNET 7:

  • medulloblastoma: by far the most common (discussed separately)
  • supratentorial PNET: 15% of all CNS PNET
    • pineoblastoma: most common supratentorial PNET  (discussed separately)
    • ependymoblastoma: also removed from the WHO in 2016
    • medulloepithelioma  (discussed separately)
  • spinal PNET: rare

The remainder of this article focuses on what has historically been denoted as supratentorial primitive neuroectodermal tumour (sPNET). 

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

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

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

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 aggressive 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 components
  • T1 C+ (Gd)
    • shows markedly heterogenous enhancement 
    • leptomeningeal seeding is common
  • DWI: often shows restricted diffusion
  • MR spectroscopy
    • elevated choline 9
    • decreased NAA 9
    • elevated taurine (Tau) peak (relatively specific for PNET) 8-9

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. 

General imaging differential considerations include:

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Article Information

rID: 5890
Synonyms or Alternate Spellings:
  • CNS primitive neuroectodermal tumour (PNET)
  • CNS PNET
  • Primary cerebral neuroblastoma
  • sPNET
  • CNS primitive neuroectodermal tumour
  • Supratentorial primitive neuroectodermal tumour
  • Supratentorial primitive neuroectodermal tumor
  • Supratentorial PNET

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