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

The purpose of this additional part of the study is to test whether the study drug, nivolumab, is a safe treatment that will prevent participants' primary central nervous system lymphoma (PCNSL) from growing again (recurrence). Participants will be people with PCNSL who continue to have cell free tumor DNA (cfDNA) in their CSF despite completion of their first treatment (first-line treatment).