Accurate detection of breast cancer region is essential for treatment. X-ray computed tomography (CT) is an effective diagnostic method of breast cancer besides MRI and ultrasound. In this paper, a semi-automated breast cancer segmentation method was proposed to CT images. First, maximum region searching was used to find the rough boundary of the lesion. Then, a modified Histogram Equalization with Iterative-Filling was adopted to enhance the lesion and avoid the unbalanced intensity in the target region. Finally, a four-seeds Random Walk was used for accurate segmentation. The method was validated on a clinical dataset with 50 cases containing 630 slices in total. The experiments showed that the Dice Coefficient of our method was 88.6%, which was higher than that of Random Walk (76.9%) and Graph-Cut (79.8%).