This patient should undergo surgical biopsy of the brain lesion without resection. The MRI is suggestive of primary central nervous system lymphoma (PCNSL), a non-Hodgkin lymphoma that can affect any part of the central nervous system but commonly presents as a focal supratentorial lesion; visual symptoms are common because the tumor often involves the optic radiations. Cerebrospinal fluid (CSF) analysis can be diagnostic in up to 10% of patients, and ocular involvement may be found in 10% to 20%. However, in most patients, including this one with negative results on CSF analysis and slit lamp examination, pathologic analysis of a brain biopsy specimen is required to confirm the diagnosis. Treatment most commonly involves methotrexate-based chemotherapy and possible whole-brain radiation.

Bone marrow biopsy is not indicated in this patient. PCNSL is a whole-organ disease but is not typically systemic. Therefore, a bone marrow biopsy for diagnostic purposes would have low yield.

Because PCNSL tends to localize to a single organ system and may have multiple associated manifestations, such as diffuse brain and ocular involvement, resection of the presenting lesion is not helpful and can actually worsen patient outcomes.

Dexamethasone and similar glucocorticoids should be avoided if possible before brain biopsy in patients with suspected PCNSL because these agents can significantly decrease the yield of the biopsy. Furthermore, this patient does not have signs or symptoms of increased intracranial pressure that would necessitate urgent glucocorticoids.

Radiation is not indicated for PCNSL until after a tissue diagnosis is made and treatment with methotrexate-based chemotherapy has begun. Additionally, when patients with PCNSL receive radiation, it must be whole-brain radiation, not focal photon-beam radiation, which is more appropriate for gliomas and other primary brain tumors.