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An intracranial tumor is a growth inside your skull, sometimes in your brain and sometimes on the covering around your brain. It can be benign (not cancer) or malignant (cancer), it can be primary (from the brain tissue itself) or metastatic (from another source in the body and traveled to the brain, like lung or breast cancer. It is always serious since the space inside the skull is limited and anything new there presses on the brain--and that's what caused the seizure. If it doesn't keep growing, it is less serious than if it is rapidly growing.
These are all questions that need to be answered by the doctors. Often an MRI will tell what exactly the tumor looks like--the type of growth it is. Rarely, they say "tumor" when they mean a different kind of mass like a fungus or other infection.
Tumors inside the cranium (skull) are often treated with radiation that goes specifically to the area but there are other treatments, like surgery or clotting off the tumor's blood supply. It all depends on what it turns out to be. Obviously, to determine precisely what it is it is good to have a tissue sample and sometimes they do get one but biopsying inside the brain area is difficult. Meanwhile controlling seizures (fits) and shrinking or destroying the tumor is what needs to be done.
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