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Objective: The aim of our study was to explore the influence of interstitial brachytherapy with 125I seeds on Cas-pase-3 and Egr-1 expressions of Lewis lung carcinoma in C57BL mice. Methods: Model mice of C57BL were planted with Lewis lung cells (LLC) and divided into the treatment group (n = 18) and the control group (n = 17). In each mouse of the treatment group, two BT-125-1 Model 125I seeds with apparent activity of 9.25 MBq were implanted into the tumor; whereas in each mouse of the control group two dummy seeds were implanted. The mice survival rates of both groups were recorded after 21 days. The tumor weights and dimensions of survived mice were measured, and the tumor volume inhibition rate was calculated. T-test was performed to compare differences of tumor weights and volumes between these two groups. Routine pathological slides of tumor tissue were observed under light microscope. The expression of Caspase-3 was detected by im-munohistochemical method and the expression of Egr-1 was detected by RT-PCR method. Results: The survival rates were 88.88% in the treatment group and 70.59% in the control group, the difference had no statistical significance (P>0.05). The tumor volume inhibition rate was 71.12%. Pathological examination showed degeneration and necrosis of cancer cells at the site nearby the seed in the treated group, but the tumor cells alive were still presented nearby the seed in the control group.The expressions of Caspase-3 and Egr-1 in the treated group were higher than those in the control group (t=12.825, P<0.01; t=7.039, P<0.01; respectively). Conclusion: The interstitial brachytherapy with 125I seeds could significantly inhibit the growth of Lewis lung carcinoma of mice. The possible mechanism may be that 125I interstitial brachytherapy can enhance the expressions of Egr-1 and Caspase-3, which could induce tumor cellular apoptosis.