Abstract： Objective To evaluate the early response rate and radiation toxicity of cyberknife in the treatment of primary or metastastic retroperitoneal tumors.Methods Twenty-eight patients with retroperitoneal tumors were treated with cyberknife.The total doses were 2000-6000 cGy ( median 4500 cGy) and biological effective doses were 3750-10080 cGy (median 7680 cGy) in 2-10 fractions (median 5).Of all patients,3 received three dimensional conformal radiotherapy (3DCRT) or intensity modulated radiotherapy (IMRT) boost,1 was treated as second-course radiotherapy,and others were treated with cyberknife only.The survival rates were calculated by Kaplan-Meier method and compared with Logrank test.Results The complete response,partial response,stable disease and progression disease rates were 43％(12/28),6％ ( 10/28),18％ ( 5/28 ),4％ ( 1/28 ),respectively.The overall response rate was 96％.The number of patients who were followed up more than 1,2,3 years were 17,9,7,respectively.The 1-,2-and 3-year local control rates were 92％,86％ and 86％,respectively.The 1-,2-and 3-year overall survival rates were 60％,49％ and 49％,respectively.The difference between local progression-free survival and overall survival was not significant ( median 9.5 and 12.0 months,x2 =0.17,P =0.680).Moreover,if the patients did not have metastasis elsewhere and local treatment was effective,there was no significant difference between local progression-free survival and progression free survival (median 17 and 11 months,x2 =0.13,P=0.720).Acute radiation-induced side effects (≥ 2 grade) such as fatigue,anorexia,nausea,vomiting and epigastric discomfort occurred in 9,9,7,7 and 2 patients,respectively.Intestinal stenosis of 1 grade occurred in 1 patient.Conclusions Radiotherapy for retroperitoneal tumors with cyberknife has provided a high response rate with minimal side effects.It is a safe and effective local treatment method for retroperitoneal tumors.