Tumefactive demyelinating lesions (TDLs), known as demyelinating pseudotumors, tumor-like demyelinating lesions, swollen demyelinating lesions, and as multiple sclerosis variants, were once classified as independent intermediate forms between multiple sclerosis and acute disseminated encephalomyelitis. It was once classified as a separate intermediate type between multiple sclerosis and acute disseminated encephalomyelitis. van Dor Velden first reported the disease in 1979. The disease often presents as a solitary mass in the central nervous system, is clinically rare, and is easily misdiagnosed as a brain tumor. The misdiagnosis rate is almost 100%. Clinical features: Acute or chronic onset. The age range is adolescent, young adult and middle age. The course of the disease is monochronic, and the disease is more severe at the onset, but gradually stabilizes with the prolongation of the disease course. The clinical manifestations are diverse. When the lesion is large, there may be brain parenchymal compression or injury, and even cranial hypertension, mental disorders and other symptoms. Ancillary tests: Most of the routine tests are not special. Cerebrospinal fluid immunoassay: IgG 24-h synthesis rate, IgG oligoclonal bands and myelin basic protein are single or multiple positive, indicating the presence of immunological abnormalities in the CNS. It helps to establish the diagnosis of demyelinating lesions. Reasons for misdiagnosis: 1. No specificity in clinical symptoms and biochemical examination; 2. Therefore, a comprehensive understanding of the clinical features and imaging manifestations can improve the preoperative diagnosis rate, and the basis for grasping the imaging manifestations is to understand the pathological changes.