What kind/size of brain tumors are not seen on ct scans? why can't they be seen? if they are small are they benign?

Many kinds: Ct exam of brain involves x-rays and for tumor detetion must have intravenous contrast injection.Some tumors are not seen, not vascular. Some tuingmors have the same density as the surround brain.Very small tumors are difficult to detect.Size is not always criteria for being benign. Certainly if no size change on subsequent scan a month or so later would suggest benign mr most sensitive for tumor.