A 56-year-old woman began to experience vertigo, headaches, and frequent falls. A computed tomography (CT) scan of the brain revealed the presence of a 1.6 x 1.6 x 2.1 cm mass involving the fourth ventricle (Figure 14.1). A gadolinium-enhanced magnetic resonance imaging (MRI) scan confirmed the presence of the mass, and a stereotactic biopsy was performed that demonstrated a primary central nervous system lymphoma (PCNSL) with a diffuse large B-cell histology. Complete blood count (CBC), lactate dehydrogenase (LDH), and beta-2-microglobulin were normal. Systemic staging with a positron emission tomography (PET)/CT scan and bone marrow biopsy showed no evidence of lymphomatous involvement outside the CNS. An eye exam and lumbar puncture showed no evidence of either ocular or leptomeningeal involvement.