Diffuse astrocytoma NOS ("protoplasmic")

Case Discussion

The patient went on to have a craniotomy and excision.

Histology

The sections show a moderately cellular glial tumor with a proliferation of mainly protoplasmic astrocytes with some intermixed fibrillary astrocytes. The protoplasmic astrocytes have enlarged round nuclei and short GFAP+ cytoplasmic processes in a background of prominent microcystic change. The fibrillary astrocytes appear more elongated and angulated. The tumor extends to involve the cerebral cortex. Sparse mitoses are seen (0-1 per 10 high power fields). There is no endothelial cell hyperplasia or necrosis. The overall features are those of diffuse astrocytoma (predominantly protoplasmic type). The Ki-67 index is about 3-4%. p53 is diffusely positive.

DIAGNOSIS: Diffuse astrocytoma of predominantly protoplasmic type (WHO grade II).

Note: IDH mutation status is not provided in this case and according to the (2021) WHO classification of CNS tumors, this tumor would, therefore, be designated as a diffuse astrocytoma NOS.

Discussion

This case illustrates very typical appearances of protoplasmic astrocytomas, a somewhat less common variant of diffuse low grade gliomas. 

Protoplasmic astrocytomas were recognized as a distinct subtype low-grade astrocytoma, however, in the 2016 update to WHO classification of CNS tumors, protoplasmic astrocytomas no longer exists as a distinct entity. 

Note the excellent T2-FLAIR mismatch sign in this case, which strongly favors astrocytoma IDH mutatant.