 Removing a tumor from your brain is hard to do because, very often, brain tumors do not have boundaries that are easy for your surgeon to find. In many cases, the surgeon cant tell exactly where the tumor begins or ends. The surgeon usually can remove most of your tumor by looking at the MRI images that were taken of your brain before surgery. However, the surgeon does not have any good way to tell if the entire tumor has been removed or not. Removing the entire tumor is very important because leaving tumor behind may allow it to grow back which could decrease your chances of survival. 