1.Disease overview: Cardiac mucinous tumor is the most common primary cardiac tumor, accounting for about 50% of cases. The majority of patients are between 30 and 50 years of age. The incidence is slightly higher in women than in men. A small number of patients have a family history. Mucinous neoplasms can occur in all atria and ventricles of the heart, and are most common in the left atrium, accounting for about 80% of cases, followed by the right atrium, accounting for about 15%, and less common in the ventricles. Most of the mucinous tumors are single foci, but very few cases present two or more tumors in the same or different heart chambers. Etiology: Mucinous tumors originate from mesenchymal cells with multi-directional differentiation potential under the endocardium. The area of the interatrial septum oval fossa is rich in such cells, so it is the preferred site. The tumor grows as a polyp-like mass into the cardiac cavity, often with a tumor tip attached to the atrial septum or atrial wall, and the tumor body can move with the cardiac cycle. The tumor is mostly oval or round, sometimes lobed or shaped like a bunch of grapes, 3~5 cm long, weighing 30~100 g. The appearance is translucent and crystalline jelly, with various colors: yellowish, light green or dark purple, interspersed with red hemorrhagic areas. It is brittle and fragile, and the broken fragments can enter the blood circulation and cause embolism of the body artery or pulmonary artery. Microscopic examination reveals a large amount of acidic mucopolysaccharide-rich stroma and a few elastic and collagen fibers, with scattered or streak-like spindle or stellate cells in the stroma. In addition, lymphocytes, plasma cells, erythrocytes, phagocytes containing iron-containing hemoglobin and smooth muscle cells were also seen. Capillaries are abundant at the base of the tumor. Mucinous tumors are mostly benign, but in a few cases they may become malignant and become mucinous sarcoma or present distant metastases. The main pathophysiological change of cardiac mucinous tumors is the obstruction of normal blood flow by the tumor protruding into the cardiac cavity. The left atrial mucinous tumor often causes obstruction of the mitral valve orifice and affects the opening and closing of the valve, resulting in mitral stenosis or incomplete closure. Clinical manifestations: Mucinous tumor with small volume can be asymptomatic. When the tumor grows, it can present three types of symptoms: hemodynamic changes, systemic manifestations and peripheral vascular embolism. The most common clinical symptoms of left atrial mucinous tumor are palpitations and shortness of breath due to obstruction of atrioventricular valve blood flow, similar to rheumatic mitral valve lesions. In more mobile mucinous tumors, sudden obstruction of the atrioventricular valve orifice may result in syncope, convulsions, or even sudden death. Mucinous tumors may also present with systemic manifestations such as recurrent fever, loss of appetite, weight loss, arthralgia, anemia, increased erythrocyte sedimentation rate, and elevated serum globulin. The mechanism of these symptoms is unclear and may be the body’s immune response to the tumor. The earliest clinical manifestation of a few cases of mucinous tumor is peripheral arterial embolism, and the diagnosis is only clear after removal of the embolus by pathological section examination. 4.Examination: Diastolic or systolic murmur can be heard in the apical region on physical examination, and the second sound in the pulmonary valve region is enhanced. In cases with greater tumor activity, the loudness and nature of the murmur may change when the patient changes position. In cases of right atrial mucinous tumor causing tricuspid valve orifice obstruction, symptoms similar to tricuspid stenosis or constrictive pericarditis may be present, such as jugular venous anger, hepatomegaly, ascites, and lower extremity edema. On physical examination, a diastolic murmur can be heard between the 4th and 5th assistants at the left border of the sternum. Left atrial mucinous tumor often shows enlarged left atrium and right ventricle, pulmonary stasis and other signs similar to mitral valve lesions on chest X-ray. The electrocardiogram is also similar to that of mitral valve lesions, but atrial fibrillation is rarely seen in cases of mucinous tumors. The clinical diagnosis of left atrial mucinous tumor is easily confused with rheumatic mitral valve lesions. Most cases of mucinous tumors do not have a history of rheumatic fever, have a short course, and the signs and symptoms may change with changes in body position. The electrocardiogram mostly shows sinus rhythm. Echocardiography may show moving clouds of light echoes of the mucinous tumor. The left atrial mucinous tumor is located in the atrial cavity during systole and migrates to the mitral valve orifice during diastole. The diagnostic accuracy of echocardiography is extremely high. Treatment and prevention: After a clear diagnosis of mucinous tumor, surgery should be performed as soon as possible to remove the tumor and restore cardiac function to avoid malignancy of the tumor and sudden death due to sudden blockage of the atrioventricular valve orifice or embolism due to tumor debris dislodgement. To remove mucinous tumor, extracorporeal circulation is used, and the tumor is removed through the left atrial incision in the interatrial sulcus, the right atrial incision, or the left and right atrial incisions, together with part of the septal tissue attached to its tip, and then the septal and atrial incisions are sutured. During the operation, attention should be paid to avoid damaging the tumor tissue. After resection of the tumor, each heart cavity should be examined in detail, and the cavity should be repeatedly cleaned with saline to prevent missing multiple mucinous tumors or residual tumor debris. Surgical treatment of mucinous tumors is effective, with low operative mortality and recurrence rates.