A brain tumor is an abnormal or uncontrollable growth of brain cells. A primary brain tumor means that the tumor originates from the brain cells themselves and is not metastasized from another part of the body. The naming and grading of a tumor depends on how the tumor tissue appears under the microscope. The name of a brain tumor is also determined by the type of brain cells from which the tumor originates. For example, a brain tumor that originates from astrocytes is called an “astrocytoma”. The grading of a tumor is determined by the degree of abnormality of the tumor cells under the microscope and how and how fast they are likely to grow and spread. Signs such as cells that are dividing (mitosis), growing faster than the blood supply, and cell death (necrosis) are indicative of active tumor growth. Tumors that are less active are usually called low-grade tumors (grade I and II tumors), while tumors that are more active are called high-grade tumors (grade III and IV tumors). The higher the grade, the higher the malignancy and the worse the prognosis.