<qa>  What is the primary central nervous system lymphoma?Background. Primary central nervous system lymphoma PCNSL is an uncommon disease. Conventional treatment has consisted of either whole brain radiotherapy WBRT or methotrexate MTX-based combined modality therapy combining chemotherapy and cranial irradiation. The treatment principles at our institute have been quite consistent in the past, sticking to the treatment protocol reported by Memorial Sloan-Kettering Cancer Center in 1990s. No matter what the dosage of MTX is, it is well-established that the addition of chemotherapy to cranial RT significantly improved survival outcomes. However, it was found that delayed treatment-related cognitive sequelae emerged as a significant debilitating complication of combined modality treatment in patients with PCNSL, especially when effective treatment can achieve disease control and better survival rates. Furthermore, the specific contribution of the disease per se and various treatment modalities to cognitive impairment remains to be clarified because the neurotoxic potential of combined modality treatments is difficult to differentiate when each can result in cognitive dysfunctions respectively. 