So what is a brain tumor? Brain tumor refers to any space occupying lesion that lies within the cranial vault. Brain tumors can be divided into those tumors that arise from the CNS tissue itself, or a tumor can also come from an organ where a tumor, such as the lung cancer, metastasized to the brain. There are nearly 20 times more brain metastases than there are primary brain cancers. Brain cancers, that is, tumors that arise from the tissue within the cranial vault itself, only make up 2 to 3% of all cancers. The most common type of brain cancer or brain tumor is that of a glioma. Another very common type of brain tumor is a meningioma, which actually comes from the linings of the brain gliomas actually come from the supporting cells of the brain. The most common type of primary brain cancer is a glioblastoma. It comprises approximately 54% of all primary brain tumors. That is, tumors that arise from the brain tissue itself. Gliomas basically are a spectrum of disease, that is, low grade gliomas, grades one and grade two are what we call low grade gliomas. Grades three and grade four are malignant gliomas. And grade three is comprised of three different subtypes: anaplastic oligodendroglioma, anaplastic astrocytoma, and anaplastic oligoastrocytoma. Grade four glioma is called glioblastoma. The grade three grade two lesions also comprise three separate lesions as well. Well differentiated astrocytoma, well-differentiated oligodendroglioma, and well-differentiated oligoastrocytoma. Typically grade one lesions such as pilocytic astrocytoma are typically seen in children.