Mediastinal nodule 21mm is not normal. Mediastinal nodule 21mm is an imaging finding that suggests a mediastinal lesion with an abnormal density shadow in the mediastinal region, approximately 21mm in size. Common diseases of mediastinum include thymoma, neurogenic tumor, lymphoma, etc. 1. Thymoma: it is a common tumor in the anterior mediastinum, mostly seen in adults, most of them are benign lesions, some of them are malignant lesions, and benign lesions have the possibility of malignancy, a small part of them are combined with myasthenia gravis, and one third of patients are asymptomatic and only found during physical examination, so it is recommended to be actively treated with surgery. 2. Neurogenic tumor: mostly located in the posterior mediastinum, most of the patients are asymptomatic, but when the volume is large, there are compression symptoms, such as chest tightness, chest pain, coughing, etc. As neurogenic tumors have the tendency of malignancy, they should be operated as early as possible after discovery. 3. Lymphoma: Lymphoma can present with enlarged lymph nodes all over the body, and the enlarged and fused lymph nodes in the mediastinum can present with 21mm nodes, which are malignant lesions and need radiotherapy treatment under the guidance of professional doctors. Therefore, patients with 21mm mediastinal nodes are recommended to go to the hospital in time for consultation, clarification of the condition and active treatment.