The patient went on to have a biopsy.

Histology

MICROSCOPIC DESCRIPTION:

The sections show a mild increase in cellularity in the white matter. Scattered astrocytes show focal crowding and uneven distribution. These cells are atypical with mildly enlarged and hyperchromatic nuclei. Mitoses are inconspicuous. No microvascular proliferation or necrosis is present. There is no oligodendroglial component. Scattered cells are p53 positive. The topoisomerase index is 1%. I

• ATRX shows no loss of staining (non-mutated) 
• MGMT is focally positive (likely unmethylated)
• IDH-1 R132H immunostain is negative
• IDH1 and IDH2 mutation status:
  • Method: Codon 132 of the IDH1 gene and codon 172 of the IDH2 gene are amplified and sequenced by pyrosequencing
  • Results: IDH1 and IDH2: No mutation detected

The features are those of diffuse astrocytoma.

DIAGNOSIS:

Diffuse astrocytoma (WHO Grade II) - gliomatosis growth pattern. 

Discussion

It is important to note that as of the 2016 revision of the WHO classification of CNS tumors, gliomatosis cerebri no longer is recognized as a diagnosis, but rather has been relegated to a growth pattern. 

Interestingly, as is in this case, IDH mutations are usually absent ¹.