To better guide the development and optimization of radiotherapy planning, to reduce the incidence of radiation reactions, and to improve the quality of life of the patients with pancreatic cancer using radiotherapy, we conducted this study to explore the dosimetric parameters that predict the risk of gastrointestinal (GI) toxicity with hypofractioned radiotherapy for pancreatic cancer.Between January 2014 and January 2015, the medical records of 68 patients with pancreatic cancer who underwent helical tomotherapy at the Air Force General Hospital were analyzed.The doses delivered to the planning target volume, clinical target volume, and gross tumor volume-internal gross tumor volume of the primary pancreatic lesions were 50, 60, and 70-80 Gy in 15-20 fractions, respectively.GI toxicity was scored according to version 4.0 of the National Cancer Institute Common Terminology Criteria for Adverse Events.The stomach and duodenum were contoured separately to determine the dose-volume histogram parameters.Univariate and multivariate analyses were adopted to identify clinical and physical risk factors associated with GI toxicity.The median follow-up was 9 months (range: 4-16 months).Eighteen patients had grade Ⅱ acute GI toxicity, one patient had grade Ⅲ acute GI toxicity, 17 patients had grade Ⅱ late GI toxicity, and one patient had grade Ⅲ late GI toxicity.On univariate analysis, the volume, the average dose Dmean, the maximum dose to 1, 3, 5, and 10 cm3 of the stomach and duodenum (D1, D3, D5, and D10),and the relative volumes receiving 5-40 Gy (V5-V40), and the absolute volumes receiving 5-45 Gy (aV5-aV45) of the duodenum were significantly associated with grade Ⅱ or higher GI toxicity (P<0.05).On multivariate analysis, aV45 of the duodenum was an independent predictor for grade Ⅱ or higher GI toxicity (P=0.031).The receiver operating characteristic analysis also showed that an aV45 of 0.5 cm3 was the optimal threshold to predict GI toxicity for the entire cohort.Our findings indicate that many dosimetric parameters of the duodenum correlate with grade Ⅱ or higher GI toxicity.To reduce GI toxicity, the absolute volume of the irradiated duodenum should be reduced.