The mediastinum is actually a gap between the sternum in the front, the thoracic vertebrae in the back (including the paraspinal rib-spine area on both sides), and the mediastinal pleura on both sides, which is connected to the neck and ends at the diaphragm. The mediastinum contains the heart, large blood vessels, esophagus, trachea, nerves, thymus, thoracic duct, abundant lymphatic tissue and connective adipose tissue. In order to easily mark the location of the lesion in the mediastinum, the mediastinum can be divided into several parts. A simple zoning method is to divide the mediastinum into upper and lower parts by using the horizontal line between the sternal angle and the lower edge of the 4th thoracic vertebra as the boundary. In recent years, the mediastinal space containing many important organs is called the “visceral organ mediastinum” (previously called the middle mediastinum); the space in front of the trachea and pericardium is the anterior mediastinum; the space behind the trachea and pericardium (including the esophagus and paraspinal mediastinum) is called the posterior mediastinum. Clinically, these two delineations are often combined to determine the site of the lesion. There are many tissues and organs in the mediastinum, and the origin of fetal structures is complicated, so there are many kinds of tumors in the mediastinum. There are primary and metastatic tumors. Among primary tumors, benign tumors are more common, but a considerable number of them are malignant. Common mediastinal tumors 1. neurogenic tumor Most of them originate from sympathetic nerves, and a few originate from peripheral nerves. These tumors are mostly located in the posterior mediastinal spine within the parasternal rib spine. Unilateral tumors are common. It is usually asymptomatic, but pain may occur when it grows to compress the nerve trunk or when it becomes malignant and erodes. Mediastinal neurogenic tumors can be divided into two major categories: (1) Tumors of the vegetative nervous system: Most of them originate from sympathetic nerves. The malignant ones are neuroblastoma and ganglioneuroblastoma, and the benign ones are ganglioneuroblastoma. There are also a few neurofibromas that occur in the vagus nerve. (2) Tumors originating from peripheral nerves: Benign tumors include nerve sheath tumors and neurofibromas. These two types of tumors are clinically similar, so they are collectively referred to as neurofibromas. Most of them occur in the spinal nerve root or its proximal segment, and a few of them come from the intercostal nerve. The malignant ones include malignant nerve sheath tumor and neurofibrosarcoma. Teratoma and dermoid cyst are mostly located in the anterior mediastinum: near the base of the heart, in front of the great vessels of the heart. Teratomas are mostly parenchymal and contain cysts of different sizes and numbers. The cyst walls are often lined with calcified sheets and contain epidermis, dermis and sebaceous glands in addition to connective tissue. The cysts are mostly filled with brownish-yellow fluid, mixed with sebum and cholesterol nodules, and with hair. The solid parts include bone, cartilage, muscle, bronchus, intestinal wall and lymphatic tissue, etc. 10% of teratomas are malignant. 3.Thymoma is mostly located in the anterior superior mediastinum: epithelial cell type, lymphocytic type and mixed type. They are oval shaped or lobulated with well-defined margins. Mostly benign, with intact envelope. However, it is often considered clinically potentially malignant and prone to infiltrate nearby tissues and organs. Myasthenia gravis is combined with myasthenia gravis in about 15% of patients, and a few patients may have simple red blood cell aplastic anemia or gammaglobulin deficiency. Conversely, about half of the patients with myasthenia gravis have thymoma or thymic hyperplasia abnormalities. Some degenerated remnant thymus glands contain active germinal centers, often vagally located in adipose tissue in the pre-tracheal, inferior pole of the thyroid, hilar, pericardial, and septal muscles. The thymus gland is involved in the body’s immune function, and some conditions may be related to altered autoimmune mechanisms. Mediastinal cyst: The more common ones are bronchial cyst, esophageal cyst (or gastrointestinal cyst, foregut cyst or intestinal-derived cyst) and pericardial cyst, all of which are caused by ectopic embryonic cells during embryonic development. All three types of cysts are benign. They are mostly round or oval in shape, with thin walls and clear marginal boundaries. 5. Intrathoracic ectopic tissue tumors and tumors of lymphatic origin: the former include post-thoracic goiter and parathyroid adenoma; the latter are mostly malignant, such as lymphatic lymphosarcoma and Hodgkin’s disease. The masses are often bilateral and irregular. Lymphogenic tumors are not suitable for surgery, but mostly treated by radiotherapy or chemotherapy. 6. Other tumors: Generally, there are mesenchymal tumors of vascular origin, adipose tissue, connective tissue and muscle tissue. It is less common. There are few positive clinical signs of mediastinal tumors. The symptoms are related to tumor size, location, growth direction and speed, texture, and nature. Benign tumors can grow to a considerable extent without symptoms or very mildly due to slow growth and growth toward the thoracic cavity. On the contrary, malignant tumor is highly eroded and progresses rapidly, so symptoms already appear when the tumor is small. The common symptoms include chest pain, chest tightness, irritation or compression of respiratory system, nervous system, large blood vessels and esophagus. In addition, some specific symptoms related to the nature of tumor may also appear. Compression of nervous system: such as Horner syndrome when compressing sympathetic nerve trunk; hoarseness when compressing recurrent laryngeal nerve; numbness of upper arm, pain in scapular region and radiating pain to upper limbs when compressing brachial plexus nerve. Dumbbell-shaped neurogenic tumors can sometimes compress the spinal cord and cause paraplegia. Irritation or compression of respiratory system: it can cause severe cough, dyspnea and even cyanosis. Tumor breaking into lung and bronchus may cause fever, pus sputum and even hemoptysis. Compression of large blood vessels: compression of the innominate vein may lead to unilateral increase of upper limb and jugular vein pressure, compression of superior vena cava may lead to swelling and cyanosis of face and upper limb, anger of superficial jugular vein, tortuous anterior thoracic vein and other signs of superior vena cava syndrome. Compression of esophagus: it can cause difficulty in swallowing. Specific symptoms: It is more significant to confirm the diagnosis, such as going up and down with the swallowing movement for retrosternal goiter; coughing up hair-like fine hairs or tofu crumb-like sebum for teratoma breaking into the lung; with severe myasthenia gravis for thymoma, etc. Diagnosis In addition to the above clinical manifestations which have important reference significance for diagnosis, the following examinations are helpful for diagnosis. X-ray fluoroscopy can observe whether the mass moves up and down with swallowing, whether there is morphological change with breathing and whether there is pulsation, etc. X-ray frontal and lateral chest radiographs can show the location, density, shape, smoothness of the edge, whether there is calcification or bone shadow of the tumor. Tomography, CT or MRI can further show the relationship between tumor and adjacent tissues and organs. If necessary, cardiovascular imaging or bronchography can further identify the interconnected parts of the tumor and the relationship with heart vessels, bronchus and lung to improve the diagnosis rate. 2.Ultrasound scan can help to identify substantial, vascular or cystic tumor. 3.Laboratory examination has certain significance for the characterization of mediastinal tumor. Serum of patients with thymoma with myasthenia gravis can detect acetylcholine receptor antibody; some patients with malignant tumor originating from germ cells (non-seminomatous cell tumor) can have increased blood β-HCG and/or AFP. 4. Radionuclide 131 iodine scan may assist in the diagnosis of retrosternal goiter. 5. Biopsy of enlarged lymph nodes in the neck can help to identify lymphogenic tumors or other malignant tumors. 6.Tracheoscopy, esophagoscopy, mediastinoscopy and other examinations can help in differential diagnosis. 7.Diagnostic radiotherapy (small dose 10-30Gy), whether it can shrink in a short period of time, helps to identify tumors sensitive to radiation, such as malignant lymphoma, etc. Treatment Except for malignant lymphogenic tumors for which radiotherapy is applicable, most primary mediastinal tumors should be treated surgically as long as there are no other contraindications. Even if benign tumors or cysts are asymptomatic, surgery is appropriate because they will gradually grow and compress adjacent organs, or even become malignant or secondary to infection. If malignant mediastinal tumor has invaded adjacent organs and cannot be removed or has distant metastasis, surgery is contraindicated and radiotherapy or chemotherapy can be given according to the nature of the pathology.