What's the likelihood of a ct scan missing a brain tumor?

Depends: If done without contrast, it could be missed. If the tumor is in the posterior fossa (back of brain or brainstem) could be missed. With a ct scan, the bones in the back of the head commonly cause artifact and a mass could be missed. If there is a concern, then a MRI with gadolinium should be performed. But a ct scan with contrast is a good study. Hope this helps.

CT not sensitive: Ct brain is a fast screening exam that is often used in the acute setting such as the emergency room. It is helpful in detecting obvious tumor and mass effect from the tumor. However, it is not sensitive in the detection of small tumor which does not produce mass effect or surrounding brain edema. Mri brain with and without contrast is the best imaging modality for detecting brain tumor.