CT scans can detect most tumors EXCEPT:

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CT imaging is good at spotting many brain tumors, but its effectiveness isn’t equal in every region. The posterior fossa, where the cerebellum and brainstem lie, is particularly challenging for CT. Dense bone around the skull base creates beam-hardening artifacts and limits soft-tissue contrast, so small or infiltrative tumors in this area can be missed or underestimated on CT. MRI, with its superior soft-tissue contrast and multiplanar capabilities, is much better for detecting and characterizing posterior fossa lesions.

In contrast, many pituitary adenomas are seen on CT when they’re large or cause bony changes, and tuberous sclerosis lesions often have calcifications that CT can pick up. Primary brain tumors typically produce mass effect or edema that CT can reveal. Therefore, CT can detect most brain tumors overall, but its limitations in the posterior fossa make that region the exception.