The patient went on to have a biopsy.

Histology

Sections show white matter infiltrated by a cellular tumor. The tumor is composed of mostly atypical medium-large sized cells with round to oval nuclei and vesicular chromatin. Some cells have prominent central nucleoli. The cells show prominent and widespread clustering and layering around blood vessels. There is plentiful apoptotic debris as well are sheets of necrotic material. There are plentiful mitotic figures. There are plentiful background lymphocytes and histiocytes.

IMMUNOHISTOCHEMISTRY: The atypical cells stain diffusely and positively with CD20, Bcl6, CD45, CD30 and MUM1. 

Bcl2 is focal and weak. CD10 and C-MYC is negative.

CD3 stains the background lymphocytes. GFAP stains background glial tissue.

Ki67 is approximately 90%.  The atypical cells show strong diffuse nuclear staining with EBER-ISH and EBV-LMP.

FINAL DIAGNOSIS: diffuse large B-cell lymphoma, non-GCB type. In this setting, this is consistent with primary CNS post-transplant lymphoproliferative disorder (PCNS-PTLD). 

Discussion

PCNS PTLD, as is the case with other primary CNS lymphomas in immunocompromised patients, is more likely to be unusual in appearance with multiple smaller masses and central non-enhancement.