Abstract: Objective To design a series of geometric indexes, which can improve the correlation between geometric parameters and dosimetric parameters. Methods 48 cases of upper abdomen were selected. Manual and automatic segmentation were performed for two organs at risk, which were stomach and duodenum. Three overlapping structures, which were the overlaps with target expanded by 5 mm, 10 mm and 20 mm, were generated for each organ at risk. The geometric parameters of overlapping structures were calculated. The relationship between these geometric parameters and the dosimetric parameters of organs was investigated. Results When the geometric parameters of overlapping structures related to the target expand 5 mm, 10 mm and 20 mm were larger than 0.4, 0.6 and 0.8 respectively, the maximum dose differences of manual and automatic segmentation were less than 3 Gy. For the case with no overlaps between the organs and the target expansions, the overlap structure corresponding to target expanding 20 mm were recommended for safety considerations. Conclusion For organs at risk in the upper abdomen, the overlapping geometric parameters were closely related to the maximum dose of organs. Overlapping geometric parameters could predict whether the difference of maximum dose caused by automaticsegmentation was clinically acceptable or not.