As expected histological evaluation was difficult due to administration of steroids. 

Histology

MICROSCOPIC DESCRIPTION:

Serial sections show cores of periventricular white matter one of which includes intact ventricular ependymal lining. All show infiltration by CD68+ monocyte-macrophages and there is prominent microglial activation. Moderate numbers of CD3+ T lymphocytes are seen forming narrow cuffs around small caliber blood vessels as well as single cells and small aggregates within intervening parenchyma. No CD20+ lymphoid cells are seen. Numerous corpora amylacea are noted.

There is moderate reactive astrocytic gliosis. No mitotic figures are identified. There is no microvascular proliferation and no necrosis is identified. Myelin appears to be decreased in density but there is no convincing evidence of plaque-like demyelination. No ground-glass oligodendrocytes are seen. There is no immunostaining for IDH-1 R132H or p53. Immunostaining of nuclei for MGMT and ATRX is intact.

Nuclear staining for topoisomerase is seen in approximately 3% of cells. These appear to be monocyte-macrophages and lymphocytes.Immunostaining for SV-40/BKV is NEGATIVE

FINAL DIAGNOSIS: 

Cerebral white matter within normal limits.

Discussion

If a diagnosis is suspected it is important that the patient does not receive steroids as it makes histological proof difficult, even if contrast does not entirely disappear. 

Nonetheless the combination of imaging features, response to steroids and absence of a diagnosis on biopsy are sufficient to make the diagnosis of CNS lymphoma with confidence.