How Can Colon Cancer Be Treated?

Cleared hep c in 2014 after Harvoni. Was told I have stage 3 compensated fibrosis. My internist said my blood work for liver disease is good...that my liver is ok. I quit drinking alcohol in April, 2013 and am glad I did. Found out about hep c in August, 2013. Developed stage 1 colon cancer March, 2017 and had colon resection in April, 2017. Temporary ileostomy reversal went well August, 2017. I worry a lot about recurrence of the colon cancer. I did not like my gastro guy and have not seen one in 2 years. My current oncologist that is following my colon cancer with blood work every 2-3 months has ordered a liver ultrasound for me in December, 2017. I hate those CT scans where they inject you with dye. I know I will have to have those now but I want to know if it’s ok for me to do one stop shopping with my oncologist instead of a gastro guy. I may have to go back to the one I don’t like and just suck it up. My oncologist can order a CT scan of my liver next June and then another ultrasound the following December. Am I being unreasonable about just letting my oncologist handle this? I was fortunate in that my hep c did not cause ascites and other bad problems associated with hep c. I am an anxiety ridden person naturally but it seems to be getting worse. Any advice?