The reasons for aggravation of hydrothorax (pleural effusion) during radiotherapy for lung cancer usually include lung tissue damage, combined infection, invasion of pleura, etc., which need to be treated promptly once discovered. 1. Radiation therapy for lung cancer may cause damage to lung tissue to a certain extent, which in turn causes inflammatory exudation of pleura or lung tissue, and in serious cases, it may also lead to symptoms such as difficulty in breathing. 2. If lung cancer is accidentally combined with lung infection in the course of treatment, it may cause inflammatory exudation, at this time, anti-infection treatment should be actively carried out to avoid further aggravation of pleural effusion. 3. When lung cancer invades the pleura, it may also cause aggravation of malignant pleural fluid, at this time, thoracocentesis should be performed to drain the fluid, so as to improve the symptom of dyspnea.