Pleural effusion often occurs in patients with malignant tumor, and the reasons are mainly the following two: 1. Tumor cells metastasize to pleura. In the late stage of malignant tumor, systemic metastasis often occurs, and tumor cells metastasizing to pleura will stimulate pleural cells to keep secreting large amount of cancerous exudate, which will collect in pleural cavity, i.e. forming pleural effusion, and sometimes cancer cells can be found in pleural effusion. 2. Hypoproteinemia. Malignant tumor patients often suffer from malnutrition due to poor appetite and huge consumption of tumor, which leads to the decrease of albumin concentration in blood, and serious hypoalbuminemia will lead to the decrease of blood colloid osmotic pressure, and water in blood will penetrate into pleural cavity and form pleural effusion. Pleural effusion in tumor patients is usually caused by a combination of the above two causes, not by a single cause. A large amount of pleural effusion will cause breathing difficulty and hypoxia. The emergency treatment is to perform thoracentesis to insert a hose into the pleural cavity to drain out the water and relieve the symptoms.