The medical term pleural effusion refers to a variety of pathological causes that cause the chest cavity to be occupied by excessive fluid, lung compression, and difficulty in oxygen entering the body, which can produce initial symptoms such as chest tightness and difficulty in breathing, requiring the release of chest fluid to relieve symptoms when necessary. Those who have pleural effusion may, in part, have cancer, a malignant neoplastic disease. There are many reasons for the occurrence of pleural effusion, such as infectious diseases (pneumonia, tuberculosis, etc.), malignant diseases (cancer, tumors of other tissues), hypoproteinemia due to various diseases, autoimmune diseases, heart failure, chest trauma thoracic hemorrhage, etc. The cause of malignant pleural effusion caused by malignant neoplastic diseases can be the pleura itself (primary) or pleural metastases from malignant tumors in other parts (secondary), and most common metastatic cancers are from lung cancer, breast cancer, ovarian cancer, gastric cancer and lymphoma, etc. The primary pleural malignancy is pleural mesothelioma. Most of the pleural effusions caused by cancer are bloody fluid. After the occurrence of pleural effusion, patients often cannot heal themselves and need to rely on some medical aids, otherwise the condition will worsen and deteriorate or even endanger their lives. Therefore, some of the factors leading to pleural effusion are caused by cancer, and most of them are not malignant diseases. However, no matter what factors lead to it, once it occurs, it is recommended to go to a regular hospital for examination and treatment and identify the cause as soon as possible, and actively treat it to prevent more serious consequences.