CAT scans are insufficient to determine if a growth is malignant in many cases. Which would lead you down the road to biopsies, which are also insufficient in many cases. Which would lead to unnecessary radiotherapy, chemo or operations in at least some cases.

If there is no way to determine whether a growth is malignant, we are fucked any way. Certainly, this is not the point of a screening test.

Screening tests are about cheaply directing further testing.

It is often possible, but not always. The problem is that for cancer even a lot of that testing is either inconclusive and/or harmful in itself. So for every false positive, and even positive tests where the tumor will never harm the patient, there is a chance of active harm being done. Including causing death.

Early screening will occasionally save some. The question is whether it will save enough to make up for that harm if we increase the frequency of testing.

CAT scans by themselves are also a pretty major radiation dose which slightly increases your risk for cancer.

Are they insufficient because of the way they work, or because their current resolution is too low ?