Introduction to Neurooncology

Abstract CNS tumors may be primary or secondary. Primary CNS tumors develop initially within the brain, spinal cord, optic nerves, cranial or spinal nerve roots, leptomeninges, and dura. Pituitary tumors are primary intracranial tumors and are sometimes classified as primary CNS tumors. Secondary tumors originate from primary neoplasms arising in organs, structures, and tissues outside the CNS and reach the CNS either by direct extension when the primary tumor is located in a contiguous structure such as the base of the skull or by metastasis, indicated by absence of continuity between the CNS tumor deposit and its primary source outside the CNS. Primary CNS tumors are generally classified according to their cellular origin (glioma, astrocytoma), their tissue of origin (meningioma, nerve sheath tumors), or their organ of origin (pinealoma). The craniopharyngioma is an exception to this scheme. Distinctive Characteristics Of CNS Tumors Although similar to systemic neoplasms in some respects, tumors involving the central nervous system have the following distinctive characteristics. • Primary central nervous system tumors rarely metastasize outside the CNS. Malignancy is indicated by rapid growth, invasion of adjacent structures, and spread to other parts of the central nervous system along anatomic pathways such as the ventricular system or subarachnoid space. Even apparently benign tumors may cause considerable morbidity or death because they involve major blood vessels or other vital structures and cannot be safely excised. Such tumors are categorized more appropriately as slowly growing rather than benign. They are sometimes described as “malignant by position.”