They tend to be accurate when something shows up. However, tumours have to be a certain size to show up on any scan. So some people go for cts and nothing shows up, then assume the scan/doctors missed something. That's usually because it was too small. It's also why a number of folk get chemo, as to mop up any cells that may be floating around the body and lymphs. It's why chemo is usually referred to as a systematic treatment.