BACKGROUND: Stereo-tactic radiation therapy (SRT) is widely used to treat intracranial diseases, but some patients suffered from radiation induced brain edema after SRT. Once radiation induced brain edema occurs,the treatment is quite difficult, and it always leads to a poor outcome. Dexamethasone has certain therapeutic effect on traumatic brain edema, but the biological mechanism is still unclear.OBJECTIVE: To observe the effect of dexamethasone on the neutrophil expression of CD18.DESIGN: A randomized control observation.SETTING: Changhai Hospital of the Second Military Medical University of Chinese PLA.MATERIALS : The experiment was carried out in Changhai Hospital of the Second Military Medical University of Chinese PLA from January 1999 to December 1999. Twenty SD rats (male and female each in half) weighing (250±50) g were used.METHODS: Twenty SD rats were divided into four groups at random. ① Blank control group (n=5): The rats were not treated without dexamethasone or irradiation; ② Irradiation group (n=5): The rats were given irradiation but no dexamethasone treatment; ③ Irradiation+1 mg/kg dexamethasone group (n=5):The rats were treated with irradiation and dexamethasone of 1 mg/kg; ④ Irradiation+5 mg/kg dexamethasone group (n=5): The rats were treated with irradiation and dexamethasone of 5 mg/kg. The heads of the rats were irradiated with 10 MeV X-ray (30 Gy), and brain tissue was removed after 2weeks to observe the pathological changes. Blood samples were taken from the carotid artery, gradient centrifugation was used, and neutrophile layer was obtained, the level of neutrophile expression of CD18 mRNA and quantity of membrane proteins in blood were detected with North blot and flow cytometry respectively.MAIN OUTCOME MEASURES: ① Blood cell count; ② Pathological results; ③ level of neutrophile expression of CD18 mRNA and quantity of membrane proteins.RESULTS: All the 20 SD rats were involved in the analysis of results without deletion. At 2 weeks after irradiation, obvious cell injury could be observed under light microscope. The level of neutrophile expression of CD18 mRNA and quantity of membrane proteins in blood were obviously increased, but the severity of cell injury was relieved in the irradiation+1 and 5 mg/kg dexamethasone groups, and the CD18 expression was markedly suppressed (P ＜ 0.05), and the suppression was more obvious in the irradiation+5 mg/kg dexamethasone group than in the irradiation+1 mg/kg dexamethasone group (P＜ 0.01).CONCLUSION: Dexamethasone can reduce the radiation induced brain edema by inhibiting the expression of CD18.