Objective: PET images are being applied in defining radiotherapy target.However,there is no recognized method for defining radiotherapy target.The aim of this study is to investigate the threshold to outline the lesion radiotherapy target on PET images and its influence factors.Methods: PET imaging for sphere lesions with different sizes(diameter: 2-50mm)under different system resolutions(FWHM: 2-9mm)in different backgrounds(background/lesion: 0,0.1,……,0.7)with different pixel sizes(FWHM/20,FWHM/19,……,FWHM/2)was simulated.The PET images were analyzed to determine the threshold [(pixel value at outline-background)/(maximum pixel value-background)] at the outline whose size equals the lesion size in different conditions.Results: The threshold changed with the lesion size and the system resolution(FWHM).It decreased sharply with increasing lesion size for the lesion diameter less than 2FWHM,reached the minimum 31%at lesion diameter=2FWHM and increased slowly with increasing lesion size for the lesion diameter more than 2FWHM,and it tended to a constant for the lesion diameter more than 8FWHM.The threshold was independent of the background.The threshold increased with increasing pixel size for the lesion diameter≤FWHM and decreased with increasing pixel size for the lesion diameter>FWHM.The relationship between the threshold and its influence factors(lesion size,FWHM and pixel size)was expressed with a formula.Conclusion: The threshold changes with lesion size,system resolution and pixel size in a certain rules.The formula could provide a method to estimate threshold to outline the lesion radiotherapy target in clinical practice.