How likely is colon cancer in 31 year old? doc said if ct normal for ruq pain then colonoscopy next. is this necessary? no other symptoms

Not common: But possible. A colonoscopy may help also finding the diagnosis, cancer or non-cancer related. There are a variety of possible colon related conditions that may be discovered and diagnosed with a colonoscopy. Always remember to ask your providers what are the goals for a specific test or procedure.

Perhaps not?: Generally speaking, a 31 year old patient with RUQ pain is unlikely to need a colonoscopy, especially if a CT is normal. It would be rare indeed for colon cancer to be causing these symptoms at your age. I assume you've had a normal abdominal ultrasound. If you haven't had an ultrasound that would be the first test to do before a CT. An online consultation at Health Tap might be helpful.