How probable is it to have chiari malformation and a brain tumor?

Combination?: Usually these are 2 separately occurring situations. A chiari 1 malformation is usually asymptomatic. It is possible that intracranial pressure from a brain tumor could cause a downward displacement of the cerebellar tonsils, complicating a chiari i malformation. This is a serious situation called herniation, and can lead to a respiratory or cardiac arrest.

Not very: Chiari malformations are fairly rare, and brain tumors are not very common either. So the combination of the two is fairly unlikely but certainly not impossible. One should be alert that a downward herniation of the cerebellum caused by a tumor is not misinterpreted as "just" a chiari malformation. For more information consult your doctors and ask for a review of your brain MRI scans.