Nivolumab Maintenance in Newly Diagnosed PCNSL With Persistent CSF Circulating Tumor DNA After Completion of First-Line Chemotherapy

Inclusion Criteria: Biospecimen and Data Collection: * Histologically documented PCNSL with the ability to submit up to 20 unstained formalinfixed, paraffin embedded (FFPE) slides from the initial tissue diagnosis prior to study registration or histologically confirmed lymphomatous involvement of the CSF and/or eye/vitreous * Patients must be able to tolerate MRI/CT scans with \[18F FDG\] * Patients must be able to tolerate lumbar punctures and/or Ommaya taps Additional Criteria for Nivolumab Maintenance Arm: All criteria in the above "Biospecimen and Data Collection" section must be satisfied as well as: * Persistent cfDNA in the CSF * Participants must be able to understand and be willing to sign a written informed consent document * Women and men who are at least 18 years of age on the day of consenting to the study * KPS ≥60 * Life expectancy of \> 3 months (in the opinion of the investigator) * Participants must have adequate bone marrow and organ function shown by: * Absolute neutrophil count (ANC) ≥ 0.75 x 10\^9/L * Platelets ≥ 75 x 10\^9/L and no platelet transfusion within the past 21 days prior to study registration * Hemoglobin (Hgb) ≥ 8 g/dL and no red blood cell (RBC) transfusion within the past 21 days prior to study registration * International Normalized Ratio (INR) ≤ 1.5 and PTT (aPTT) ≤ 1.5 times the upper limit of normal. Patients on anticoagulants should be managed according to MSK SOPs as outlined in: 'Therapeutic Anticoagulation Guidelines …