Objective:To compare the differences of actual absorbed doses of liver malignant tumors after 90Y-selective internal radiation therapy (SIRT) evaluated by 90Y PET/CT and 90Y bremsstrahlung (BRS) SPECT/CT imaging, and to compare the image quality of the 2 imaging methods. Methods:Twenty-one patients (15 males and 6 females; age: (52.4±15.4) years) with liver malignant tumors (15 cases of primary liver cancer, 6 cases of liver metastases; 39 lesions) between September 2021 and August 2022 were retrospectively analyzed. All patients underwent both 90Y PET/CT imaging and 90Y BRS SPECT/CT imaging in the Department of Nuclear Medicine, Hainan Cancer Hospital. The ratios of the actual absorbed doses based on 90Y PET/CT imaging and 90Y BRS SPECT/CT imaging to the lowest standard absorbed dose(100 Gy) for tumor response were calculated. The image contrast and distinguishability of the two imaging methods were scored. Wilcoxon signed rank test and Wilcoxon rank sum test were used for data analysis. Results:The tumor absorbed doses evaluated by 90Y PET/CT and 90Y BRS SPECT/CT were 143.94(55.91, 233.48) Gy and 107.82(53.59, 157.53) Gy respectively. The doses evaluated by 90Y PET/CT were higher than the standard threshold in 24 lesions, while 19 lesions showed higher evaluated doses by 90Y BRS SPECT/CT than the standard threshold. Compared with 90Y PET/CT, 90Y BRS SPECT/CT underestimated the tumor absorbed dose of -24.25%(-32.32%, -12.14%). The ratio of dose evaluated by 90Y PET/CT to the lowest standard threshold was 1.33(0.56, 1.91), which was higher than that of dose evaluated by 90Y BRS SPECT/CT to the lowest standard threshold (0.97(0.47, 1.25); z=0.04, P<0.001). PET/CT image contrast was scored 0, 1, 2, 3 in 2, 2, 12, 23 lesions respectively, and SPECT/CT image contrast was scored 0, 1, 2, 3 in 2, 3, 16, 18 lesions respectively ( z=-1.29, P=0.199). The distinguishability scores of 0, 1, 2 based on PET/CT images were found in 3, 15, 21 lesions, while those based on SPECT/CT images were found in 4, 32, 3 lesions respectively ( z=-2.79, P=0.005). Conclusion:90Y PET/CT imaging is superior to 90Y BRS SPECT/CT imaging in radiation dose evaluation and tumor focus differentiation in patients with liver malignant tumors after 90Y-SIRT.