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
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 agressive 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 heterogenous contrast enhancement
Non specific and range from avascular to hypervascular 6.
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
- 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.
General imaging differential considerations include
- 1. Chen AY, Lee H, Hartman J et-al. Secondary supratentorial primitive neuroectodermal tumor following irradiation in a patient with low-grade astrocytoma. AJNR Am J Neuroradiol. 2005;26 (1): 160-2. AJNR Am J Neuroradiol (full text) - Pubmed citation
- 2. Shogan P, Banks KP, Brown S. AJR teaching file: Intraventricular mass. AJR Am J Roentgenol. 2007;189 (6): S55-7. doi:10.2214/AJR.07.7027 - Pubmed citation
- 3. Mitchell CS, Wood BP, Shimada H. Neonatal disseminated primitive neuroectodermal tumor. AJR Am J Roentgenol. 1994;162 (5): 1160. AJR Am J Roentgenol (citation) - Pubmed citation
- 4. Zagzag D, Miller DC, Knopp E et-al. Primitive neuroectodermal tumors of the brainstem: investigation of seven cases. Pediatrics. 2000;106 (5): 1045-53. doi:10.1542/peds.106.5.1045 - Pubmed citation
- 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
Synonyms & Alternative Spellings
|Synonyms or Alternative Spelling||Include in Listings?|
|CNS primitive neuroectodermal tumour (PNET)||✗|
|Primary cerebral neuroblastoma||✗|
|CNS primitive neuroectodermal tumour||✓|
|Supratentorial primitive neuroectodermal tumour||✗|
|Supratentorial primitive neuroectodermal tumor||✗|