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.
Please refer to embryonal tumours with multilayered rosettes (ETMR) for a current discussion.
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 tumours 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
- 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 recognised as a histological pattern 10,11.
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.
Supratentorial PNETs, by definition, occur above the tentorium and can either arise in the cerebral hemispheres or within the lateral ventricles.
- 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 heterogeneous contrast enhancement
Non-specific and range from avascular to hypervascular 6.
The extent of peritumoral 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
- 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
- elevated choline 9
- decreased NAA 9
- elevated taurine (Tau) peak (relatively specific for PNET) 8-9
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.
General imaging differential considerations include:
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- 5. Majós C, Alonso J, Aguilera C et-al. Adult primitive neuroectodermal tumor: proton MR spectroscopic findings with possible application for differential diagnosis. Radiology. 2002;225 (2): 556-66. doi:10.1148/radiol.2252011592 - Pubmed citation
- 6. Altman N, Fitz CR, Chuang S et-al. Radiologic characteristics of primitive neuroectodermal tumors in children. AJNR Am J Neuroradiol. 6 (1): 15-8. AJNR Am J Neuroradiol (abstract) - Pubmed citation
- 7. Newton HB, Jolesz FA. Handbook of neuro-oncology neuroimaging. Academic Pr. (2008) ISBN:012370863X. Read it at Google Books - Find it at Amazon
- 8. Kovanlikaya A, Panigrahy A, Krieger MD et-al. Untreated pediatric primitive neuroectodermal tumor in vivo: quantitation of taurine with MR spectroscopy. Radiology. 2005;236 (3): 1020-5. doi:10.1148/radiol.2363040856 - Pubmed citation
- 9. Chawla A, Emmanuel JV, Seow WT et-al. Paediatric PNET: pre-surgical MRI features. Clin Radiol. 2007;62 (1): 43-52. doi:10.1016/j.crad.2006.09.008 - Pubmed citation
- 10. Louis DN, Perry A, Reifenberger G et-al. The 2016 World Health Organization Classification of Tumors of the Central Nervous System: a summary. Acta Neuropathol. 2016;131 (6): 803-20. doi:10.1007/s00401-016-1545-1 - Pubmed citation
- 11. Louis DN, Ohgaki H, Wiestler OD, Cavenee WK "WHO Classification of Tumours of the Central Nervous System. 4th Edition Revised" ISBN: 9789283244929