Astroblastoma, MN1-altered

Astroblastomas are rare glial tumours usually found in the cerebral hemispheres of young adults and children. 

Epidemiology

They occur at all ages range from early childhood to 6th decade but are most commonly seen in children, adolescents, and young adults with a mean age between 10-30 years ^2,5. A female predilection has been suggested in some reports ⁵. 

Pathology

Astroblastomas, along with choroid gliomas of the third ventricle and angiocentric gliomas, are grouped as other neuroepithelial tumours in the current (2016) version of the WHO classification of CNS tumours ⁵.

They have not yet been given a WHO grade, but do have a range of histological appearance and biological behaviour ranging from relatively indolent (astroblastoma) to aggressive (anaplastic or malignant astroblastoma) ^4,5. 

Macroscopic appearance

They are well-demarcated masses with areas of cystic degeneration and necrosis giving it a bubbly appearance. Haemorrhage and adjacent brain infiltration are rare.

Microscopic appearance

Astroblastomas have elongated glial cells with abundant eosinophilic cytoplasm and a broad, or “stout”, GFAP positive process extending radially towards a central blood vessel, forming "astroblastic pseudorosettes", reminiscent to the perivascular pseudorosettes of ependymomas, which are a characteristic feature of this tumour. The central vessel tends to be hyalinized. 

Although no agreed-upon criteria have been accepted, anaplastic/malignant histological features include ⁵: 

• increased mitotic activity (>5 mitoses per 10 high-powered fields)
• high cellularity 
• anaplastic nuclear features
• microvascular proliferation
• palisading necrosis 
• usually Ki-67 >10%

Immunophenotype

• GFAP: positive
• S100: positive
• vimentin: positive
• EMA (cell membrane): variable

Radiographic features

Astroblastomas are usually fairly sizable, peripherally located, supratentorial lobulated solid cystic masses with little if any associated vasogenic oedema ^1-5. Multiple cysts are common and can give it a bubbly appearance. Calcification is very common and seen in 85% of cases, and usually in a punctate pattern.

MRI

• T1: iso- to hypointense
• T2/FLAIR: heterogeneously hyperintense
• T1 Gd (C+): heterogeneous enhancement

Treatment and prognosis

Surgical resection is the treatment of choice with adjuvant radiation therapy and chemotherapy reserved for high-grade lesions. Recurrence after complete surgical resection of low-grade tumours is uncommon ⁴. 

Differential diagnosis

Possible imaging differential considerations include:

• supratentorial ependymoma
• atypical teratoid/rhabdoid tumour
• astrocytoma, particularly GBM or gemistocytic
• oligodendroglioma
  • nodular calcifications instead of punctate calcifications ²
• pleomorphic xanthoastrocytoma