Best to ask your consultant for a definitive answer.

In my own case, I had a gastroscopy and a biopsy before I had the CT scan which they said was more accurate for establishing the position and orientation of my primary cancer and for locating all my secondaries. 

Not being able to have a colonoscopy isn’t ideal but the treatment seems likely to be about the same and about as effective without a biopsy. You definitely need to talk to a medical expert though, I’m just another patient.

Best wishes

Dave