Astroblastoma, MN1-altered

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

Terminology

The exact nature of astroblastomas remains to be fully elucidated. Tumors with compatible histological features have a variety of molecular characteristics and overlap with other better-known tumors such as pleomorphic xanthoastrocytomas or ependymomas or diffuse astrocytomas ^6,7. 

The presence of MN1 alterations (a transcriptional coregulator involved in the development of acute myeloid leukemia and meningiomas) is now used to define the lesion,  and thus it is now named astroblastoma, MN1-altered and considered circumscribed astrocytic gliomas in the current (5th Edition, 2021) WHO classification of CNS tumors ⁶.

Epidemiology

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

Pathology

Astroblastomas have not yet been given a WHO grade, but do have a range of histological appearance and biological behavior 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. Hemorrhages 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 tumor. 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 edema ^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

Reported signal characteristics include ²:

• T1: iso- to hypointense
• T2/FLAIR
  • ​heterogeneous with a multicystic or bubbly appearance
  • solid components tend to be isointense to grey matter
  • relatively little surrounding edema compared to size
• T1 C+ (Gd): heterogeneous enhancement
• DWI/ADC
  • ​facilitated diffusion in fluid/cystic components
  • intermediate ADC values of the solid component (1190 to 1250 x 10^-6  mm²/s) ⁸

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 tumors is uncommon ⁴. 

Differential diagnosis

Possible imaging differential considerations include:

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