Objective: This study is to determine whether dual phase C-11 acetate PET imaging pattern can be used in the differential diagnosis of hepatic tumors.Methods: 36 patients with hepatic tumors were selected to enroll in this study (26 men, 10 women).ALL patients underwent ultrasound, contrast CT and MRI studies.Paradox or undetermined diagnosis were found in 30 patients after these examinations.32 patients underwent liver resection and 4 patients received biopsy 1-2 weeks after acetate PET examination.Malignant tumors were given further grade classification.Dual phase PET studies were performed on each patients, including initial upper abdomen images(starting with injection and lasting about 10min)and delayed whole body scan (10min after injection and lasting about 15-50min).Region of interest (ROI) technique was performed for semi-quantitative analysis.Tumor-to-liver ratio (T/B,T/B =SUVmax of the lesion/SUVmax of nontumor liver tissue) was calculated for comparative analysis.Results Totally 37 lesions (14 benign and 23 maglignant) were proved by pathological analysis.3lesions (2 FNHs and hemingioma) demonstrated intensively high uptake of acetate in initial PET images and then decreasing with time passing by.19 lesions showed as persistent hot spots in dual phase imaging which were 17 HCC (1 poor differentiated HCC nodule, 16 well and medium HCC nodules), 1 reactive hyperplasia lymph node and 1 angiomyolypoma.SUV of HCC showed irrelative to pathological grade classification, but T/B ratios of HCC, which were from 1.5 to 2.8,increased in delayed images comparing to initial ones.15 lesions presented as photopenic area in dual phase PET imagings.Pathology proved that they were 7 malignant nodules (2 poor differentiated HCC, 4 adenocarcinoma and 1 sarcoma) and 8 benign tumors (all cirrhosis nodules and inflammatory psedo tumors).The sensitivity, specificity, accuracy of dual phase acetate PET imaging in the differentiated diagnosis of benign and malignant hepatic tumors were 70.8%,84.6% and 75.7% respectively.Positive and negative predictions were 89.5% and 61.1%.Conclusions: Dual phase acetate PET imaging is a promising technique to be used as simple and effective method in clinical practice to differentiate some special benign hepatic tumors from malignant ones, especially FNH from HCC.However, imaging features of benign and malignant hepatic tumors are still overlap with each other.