Thymoma: Thymoma is one of the most common mediastinal tumors and is a group of diseases with distinctive clinicopathological features and multiple paraneoplastic symptoms derived from different thymic epithelial cells. Thymoma is an important immune origin of the human body that originates from the endoderm of the 3rd (or 4th) gill arch during the embryonic period and is a derivative of the primitive foregut epithelial cells that attach to the anterior mediastinum as the embryo grows and develops. Thymic tumors originating from thymic epithelial cells or lymphocytes are the most common, accounting for 95% of thymic tumors and ranking 1st to 3rd in the whole mediastinal tumors. In a group of 4,968 mediastinal tumors in Japan, thymoma ranked third after teratoma and neurogenic tumors, accounting for 22.37%. Clinical manifestations: Like any mediastinal tumor, the clinical symptoms of thymoma arise from the compression of surrounding organs and the symptoms specific to the tumor itself – combined syndrome. Small thymoma mostly has no clinical complaints and is not easily detected. When the tumor grows to a certain size, the common symptoms are chest pain, chest tightness, cough and anterior chest discomfort. The nature of chest pain is not characteristic, the degree is different and the location is not specific, generally speaking, it is mild and often treated symptomatically without further examination. If the symptoms persist for a long time, some patients may undergo X-ray examination, or some patients may find mediastinal mass shadows during chest X-ray or chest radiography. The overlooked thymoma often grows to a large size at this time, compressing the veins or showing signs of superior vena cava obstruction syndrome. Severe chest pain, rapid worsening of symptoms within a short period of time, severe irritating cough, dyspnea due to pleural effusion, shortness of breath due to pericardial effusion, and skeletal pain of peripheral concern all suggest the possibility of malignant thymoma or thymic carcinoma. The specific manifestation of thymoma is the combination of certain syndromes, such as myasthenia gravis (MG), pure red blood cell aplastic anemia (PRCA), hypoglobulinemia, nephritis nephrotic syndrome, rheumatoid arthritis, dermatomyositis, lupus erythematosus, and megalophagia. Pathological changes: Pathologically thymoma is named after the cellular component that accounts for more than 80% of the cells. They are divided into epithelial cell type and mixed epithelial cell lymphocyte type. It is difficult to distinguish benign or malignant thymoma from pathological morphology alone. Based on clinical manifestations, what is seen by the naked eye during surgery and pathological morphological features, it is more appropriate to classify invasive and non-invasive thymomas. However, it is customary to refer to benign and malignant thymomas. The differentiation between benign and malignant thymoma A is based on clinical presentation and surgical findings. During surgery, attention should be paid to: ① whether the tumor has an intact envelope; ② whether the tumor has aggressive growth; ③ whether there is distant metastasis and intra-thoracic implantation; ④ the anomaly of cell morphology under microscope, in order to reach the correct conclusion. If the tumor has an intact fiber envelope at the time of surgery, the tumor grows within the envelope and does not infiltrate the surrounding organs with adhesions, and can be easily removed by surgery, it is benign or non-invasive thymoma. When the tumor invades out of the envelope and invades the surrounding organs or tissues (pericardium, pleura, lung and blood vessels, etc.), and cannot be removed by surgical operation or cannot be completely removed, or if intra-thoracic implantation or pleural metastasis is found at the time of operation, it is considered malignant or invasive thymoma. Diagnosis: X-ray examination is an important method to detect and diagnose mediastinal tumor. In the orthopantomogram, thymoma often appears as a round or oval dense shadow with one septum widened or protruding to one side of the chest cavity, more to the right than to the left, or to both sides of the chest cavity. The protrusion to the left is often obscured by the aortic bulb, while the protrusion to the right may overlap with the superior vena cava. The edges of the mass are clear and sharp, and some of them are lobulated. The lateral image shows a substantial mass shadow with uniform density in front of the great vessels of the heart located behind the sternum. A small number of thymomas show striated, punctate, massive and shapeless calcifications, which are less calcified than teratomas. Some thymomas are flattened over the large blood vessels of the heart and are the most difficult to diagnose on x-ray. Lateral lesion tomography is a simple and economical way to identify thymoma, showing the presence, size, and density of the tumor, and is particularly useful when complex examinations are not available. Chest CT is an advanced and sensitive method to examine mediastinal tumors. It can accurately show the location, size, protrusion to one side or both sides, tumor margins, the presence or absence of surrounding infiltration, and the determination of surgical resectability. Pathological biopsy is necessary to take biopsies for histological classification before treatment, because there are many kinds of mediastinal tumors, simple method is to do cytological examination by needle prick, or special empty needle puncture is better to take histological classification. If necessary, open-chest exploration for frozen histological examination is necessary to determine whether surgery can be performed at the same time. Common lesions that need to be differentiated from thymoma include teratoma and ascending aortic aneurysm. Teratomas often occur in young and middle-aged adults and may be asymptomatic or may have recurrent pulmonary infections, sometimes with a history of coughing up hair or grease-like material, and may have calcified teeth or bone within the mass on x-ray. It is reported in the literature that mediastinal tumors are mistaken for ascending aortic aneurysms, or ascending aortic aneurysms are misdiagnosed as thymic aneurysms. In the lateral phase of the chest, the ascending aortic aneurysm appears as a round shadow along the left ventricle, and the mass can be seen on chest X-ray as a distending pulsation. In recent years, magnetic resonance imaging (MRI) has been gradually increasing in clinical application and has special value for the diagnosis of cardiac macrovascular malformation3 and hemangioma, and is a sensitive and effective examination method to distinguish mediastinal tumor from ascending (descending) aortic aneurysm. Treatment principle: Thymoma should be surgically removed as soon as it is diagnosed. The reason is that the tumor continues to grow and enlarge, compressing the neighboring tissues and organs to produce obvious clinical symptoms; it is difficult to judge the benignity and malignancy of the tumor simply from clinical and X-ray performance; moreover, benign tumor can also become malignant. Therefore, no matter benign or malignant thymoma, it should be removed as soon as possible. For partially resected malignant thymoma, postoperative radiation therapy can relieve the symptoms and prolong the patient’s survival.