Treatment with bevacizumab is standard of care for recurrent high grade gliomas (HGGs) and the level of border distinctness is a major parameter to monitor the therapy. Previously, the level of border distinctness was defined qualitative. In this study, we calculated the distinctness of the fluid-attenuated inversion recovery (FLAIR) hyperintense border—edge contrast (EC)—and showed it improves the evaluation of response to bevacizumab in patients with HGG. We showed that after bevacizumab, lower EC of the FLAIR hyperintense region was associated with poorer survival among HGG patients. We developed a quantitative parameter to characterize the border of the tumor.