Protoplasmic astrocytoma (historical)
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Protoplasmic astrocytoma is a historical term previously applied to an uncommon variant of diffuse adult-type astrocytomas. In the updated 4th edition (2016) of the WHO classification of CNS tumors, protoplasmic astrocytomas were removed as a distinct entity 6.
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.
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.
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, more recently 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
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