My cancer doctor is doing a ct scan why? is she doing it to see if the cancer is back?

CT very useful: CT scans are often very useful to check to see if cancer has returned (as you had guessed). So getting ct scans periodically, such as every 6 months, or every year is Common, and is an excellent tool for this reason. CT is also helpful to determine the extent of disease, which organs are involved, and if cancer has spread or enlarged. They also help to see if it has responded well to treatment.