Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Brain Tumors ?
    ### Response:
    A brain tumor is a growth of abnormal cells in the tissues of the brain. Brain tumors can be benign, with no cancer cells, or malignant, with cancer cells that grow quickly. Some are primary brain tumors, which start in the brain. Others are metastatic, and they start somewhere else in the body and move to the brain.
    