Protoplasmic astrocytoma is a historical term previously applied to an uncommon variant of diffuse adult-type astrocytomas.
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Terminology
The term was removed in the updated 4th edition (2016) of the WHO classification of CNS tumors, with these tumors folded into what is now referred to as astrocytoma, IDH-mutant 6.
Pathology
Protoplasmic astrocytomas were composed of neoplastic astrocytes with rounded prominent nuclear contours and little cytoplasm. They had scant processes. The tumor matrix contained numerous and prominent microcystic spaces filled with mucinous fluid, that presumably accounts for their T2/FLAIR appearance 3.
Radiographic features
CT
Protoplasmic astrocytomas typically appeared as hypodense regions of positive mass effect, usually without any enhancement. Areas of the tumor appeared of near fluid attenuation, due to the aforementioned prominent mucinous microcystic component.
MRI
These tumors had fairly characteristic appearances 4:
T1: hypointense compared to white matter
T2: strikingly hyperintense
FLAIR: large areas of T2 hyperintensity suppressing on FLAIR - these are not macrocystic but rather represent the areas with abundant microcystic change - and is known as T2/FLAIR mismatch sign, shown to be predictive of absence of 1p19q co-deletion
T1 C+ (Gd): usually little or no enhancement
MR spectroscopy: elevated choline:creatine ratio
MR perfusion: there is reduced rCBV