Cystic hypodense shadow in the left anterior superior mediastinum is an imaging finding, which may be localized occupying lesions, such as thymic tumors, lymph node enlargement, adolescents, or undegenerated remnants of thymic tissue. 1. Thymic tumor: cystic low-density shadow in the left anterior superior mediastinum may be caused by benign tumor of thymic tissue or malignant tumor and other related diseases. 2. Enlarged lymph nodes: due to local inflammation and infection, there may be enlarged lymph nodes, or the tumor metastasizes to the lymph nodes, which may appear as cystic low-density shadow in the left anterior superior mediastinum. 3. Undegraded residual thymic tissue: If adolescents find cystic hypodense shadow in the left anterior superior mediastinum, it may belong to undegraded residual thymic tissue. When cystic hypodense shadow is seen in the left anterior superior mediastinum, there may be other reasons, and it is necessary to go to the hospital in time to further improve the relevant examinations, to clarify the cause of the disease, and actively treat it, so as to avoid delaying the condition.