Fast contouring is important in image-guided radiation therapy (IGRT) and adaptive radiation therapy (ART) where large computed tomography (CT) volumes have to be segmented. In this study, a modified active contour (also called snake) segmentation method based on a faster gradient-vector-flow (GVF) calculation algorithm is proposed. The accelerated method was tested on multiple organs, including lung, right ventricle, kidney and prostate. Compared to the original algorithm, the improved one reduced GVF calculation times to one-half or less without compromising contour accuracy.