1. Primary CNS lymphoma refers to CNS lymphoma of lymphocytic origin without lesions outside the CNS. This disease is rare. The incidence accounts for 1% of all intracranial tumors and 2% of all malignant lymphomas. 2. Most of the lymphomas occurring in the CNS are non-Hodgkin’s lymphomas and are mostly B-cell in nature. The etiology is still unclear and may be related to immune deficiency. The disease progresses rapidly. 3. Tumors are found in the deep white matter of the brain, mostly in the frontal and temporal lobes, the basal ganglia of the thalamus and the cerebellum. The tumor is low signal or isosignal in T1WI, isosignal or slightly high signal in T2WI, and the tumor is obviously uniformly reinforced with clear border after enhancement. The peri-tumor edema is mild to moderate. The edema zone of superficial tumor is more obvious, while the edema zone of deep tumor is lighter. The most characteristic change of these tumors is that the size of the tumor is disproportionate to the peritumoral edema occupying effect. This feature is especially obvious in tumors located in deep brain, and this feature is often used as a differentiation point with other intracranial tumors. Histopathological diagnosis is the only effective method to confirm the diagnosis of tumor. 5. The purpose of gamma knife treatment is to control the development of the lesion and prolong the survival of the patient. Primary central nervous system lymphoma is mostly sensitive to radiation, the use of gamma knife treatment can firstly irradiate the tumor with high dose in a short time, rapidly shrink the tumor, reduce intracranial pressure and improve clinical symptoms. Secondly, because the high-dose radiation beam is mainly located in the visible tumor site during gamma knife treatment, the surrounding normal brain tissue receives less dose and less damage. Therefore, in recent years, primary central nervous system lymphoma mostly adopts the integrated treatment method of gamma knife + chemotherapy + whole brain radiotherapy.