We describe a case of CD10-positive PCNSL presenting with multipleposterior fossa enhancing lesions in an immunocompetent older woman with ahistory of breast cancer successfully treated by the RTOG 0227 protocolconsisting of pre-irradiation chemotherapy with high-dose methotrexate,rituximab, and temozolomide for 6 cycles, followed by low-dose whole-brainradiation and post-irradiation temozolomide. The gene discussed is MME; the disease is primary central nervous system lymphoma.