A patient with stage IV diffuse large B-cell lymphoma with BCL2::IgH and MYC::ZCCHC7 fusions was treated with Pola-R-CHP and high-dose methotrexate to achieve a complete response. He developed memory loss and a change in balance one year later. PET/CT and MRI head showed leptomeningeal and cranial nerve disease. CSF cytology showed large lymphoid cells with basophilic cytoplasm. They were CD45int/SSCint, CD34-CD117-CD19-CD20+ by flow cytometry. IGH PCR was positive for heavy chain gene rearrangements. He was diagnosed with secondary CNS lymphoma.