I'm just wondering, if you had stomach cancer would doctors see it on a CT scan of the abdomen?

Not unless it . Is advanced case. Gastric cancer, as other GI malignancies, start in the inner lining of a hollow organ, so in its early stages can be difficult to visualize on ct scan. By the time gastric, and other GI cancers, become visible on ct, they are far advanced. So don't rely on negative ct scan to exclude the possibility of stomach or colon tumors.