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Background Prevention is presently the only available method to limit radiation-induced lung morbidity. A good predictor is the key point of prevention. This study aimed to investigate if [18F]2-fluoro-2-deoxyglucose (FDG) uptake changes in the lung after radiotherapy could be used as a new predictor for acute radiation pneumonitis (RP). Methods Forty-one patients with lung cancer underwent FDG positron emission tomography/computed tomography (FDG-PET/CT) imaging before and after radiotherapy. The mean standardized uptake value (SUV) was measured for the isodose regions of 0-9 Gy, 10-19 Gy, 20-29 Gy, 30-39 Gy, 40-49 Gy. The mean SUV of these regions after radiotherapy was compared with baseline. The mean SUV in patients who developed RP was also compared with that in those who did not. The statistical difference was determined by matched pair t test. The Radiation Therapy Oncology Group (RTOG)criteria were used for diagnosis and grading of RP.Results With a median follow-up of 12 months, 11 (26.8%) of the 41 patients developed grade 2 and above acute RP.The mean SUV of regions (10-19 Gy, 20-29 Gy, 30-39 Gy, 40-49 Gy) increased after radiation therapy in all 41 patients.The mean SUVs after radiation therapy were 0.54, 0.68, 1.31, 1.74 and 2.27 for 0-9 Gy, 10-19 Gy, 20-29 Gy, 30-39 Gy and 40-49 Gy, respectively. Before the radiation therapy, the mean SUV in each region was 0.53, 0.52, 0.52, 0.53 and 0.54, respectively. These patients had significantly higher FDG activities in regions receiving 10 Gy or more (P <0.001).Compared with their counterparts, the elevation of SUV was significantly greater in those patients who developed acute RP subsequently.Conclusion The mean SUV of the lung tissue may be a useful predictor for the acute RR FDG-PET/CT may play a new role in the study of the radiation damage of the lung.