Primitive neuroectodermal tumor of the CNS

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

Please refer to embryonal tumors with multilayered rosettes (ETMR) for a current discussion.

Terminology

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

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

  • 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 and incorporated into ETMR
    • medulloepithelioma  (discussed separately)
  • spinal PNET: rare

Additionally, a primitive neuronal component can also be identified in glioblastomas, previously referred to as glioblastoma with PNET-like component but now simply recognized as a histological pattern .

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

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

Epidemiology

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

Clinical presentation

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.

Pathology

Supratentorial PNETs are composed of primitive undifferentiated neuroepithelial cells and are WHO grade IV tumors. They are small round blue cells, malignant embryonal tumors 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

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
  • cystic components are common (≈65% )
  • calcification can be common (≈70% )
  • shows heterogeneous contrast enhancement
Angiography

Non-specific and range from avascular to hypervascular .

MRI

The extent of peritumoral vasogenic edema is surprisingly low given the size of the mass and the aggressive tumor characteristics .

  • 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 heterogeneous enhancement 
    • leptomeningeal seeding is common
  • DWI: often shows restricted diffusion
  • MR spectroscopy
    • elevated choline
    • decreased NAA
    • elevated taurine (Tau) peak (relatively specific for PNET)

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: