Can a ct scan miss a brain tumor or mass?

Not likely: A large brain tumor or mass would not be missed by ct scan. A smaller or vascular lesion could be missed and an MRI is in order if you are symptomatic.

Yes: Ct scans can certainly on occasion miss a small tumor, especially when they are down in the posterior fossa at the base of the skull. Mri scans are more sensitive and less likely to miss a tumor, but even they can on rare occasions miss one that shows up later. These scans are read by human beings, and they can miss something that is there, too.