[en] Objective: To evaluate the value of the ¹⁸F-fluorodenxyglucose (FDG) PET/CT in malignant tumors after gamma knife treatment. Methods: Eighteen patients with malignant tumors after gamma knife treatment were examined by PET/CT. The imaging manifestation of PET and CT was analysed. And the pre- and post-treatment changes of PET and CT were compared. Results: The effective changes in CT after therapy: shrinkage, fibrosis of the focal lesions, low density liquefied central necrosis in large lesions with patchy calcification. The result evaluated by CT analysis: complete remission (CR) 3 cases (16.7%), partial remission (PR) 10 cases (55.6%), no change (NC) 2 cases (11.1%), progress disease (PD) 3 cases (16.7%); CR + PR 72.2%. The effective changes of 34 lesions in PET: less concentrated, vague, unclear border, uneven hot areas with defect in necrotic areas. The result evaluated by PET: CR 4 cases (22.2%), PR 8 cases (44.4%), NC 1 case (5.6%), PD 5 cases (27.8%), CR + PR 66.7%. Standardized uptake value (SUV) was changed in all 34 lesions after treatment. There were 29/34 lesions decreased and 5 lesions increased. The change of ratio of SUV was greater than 50% in 15 lesions. The SUV in pre- and post-treatment group was 6.33 ± 2.62 and 3.88 ± 1.82, respectively with statistical difference between them (t=5.580, P<0.001). Conclusions: PET/CT has important value in the evaluation of local control of tumor growth after the gamma knife treatment in malignant tumors. The detection of residual and recurrent tumors in PET is apparently better than CT. (authors)