Abstract: Objective : To assess the clinical performance of a integrated PET/CT system using ¹⁸ FDG in patients with malignant tumors and determines the impact of PET/CT on staging and managing patients for radiotherapy. Methods : One hundred and forty -eight consecutive cases of patients with various malignant tumors were analyzed. The Whole -body PET/CT was performed before radiotherapy and the comparison was conducted on a lesion -by -lesion basis. The lesions determined by PET/CT were compared with physical examination and conventional radiography findings. The strategy of radiotherapy before and after PET/CT scanning was compared and the change in the treatment management determined. Results : A total of 79 cases with malignant or equivocal lesions that were not previously detected in the basis of physical examination and conventional radiography were newly found by PET/CT. The diagnoses of 25 malignant lesions were modified because no lesion was found or no FDG concentration was observed on PET/CT imaging. In 28 patients (18.9%) the stage were changed due to PET/CT results. In 57 patients (38.5%) management in radiotherapy were changed due to PET/CT results. In 18 patients (12.2%) radiotherapy was not performed after PET/CT. In 10 patients (6.8%) the intention of radiation treatment (curative/palliative) were changed due to PET/CT results. Correction of radiation dose was made in 17patients (11.5%). Correction of the target volume was made in 28 patients (18.9%). Conclusion : The Whole -body PET/CT performed before radiotherapy increases the accuracy of the diagnosis and the definition of the orientation, and decreases the focus omitted. It is valuable to direct the treatment planning of radiotherapy.