I. Introductory Concepts
A. Tumor Classification
1. Tumors may be primary, originating in cells found in the brain and its coverings, or secondary, that is, metastatic from sites outside the CNS, or developmental, arising from displaced midline epithelium or germ cells.
2. Tumors may be classified as intra-axial, within the brain parenchyma, or extra-axial tumors originating in skull, meninges, cranial nerves and brain appendages such as pituitary gland.
3. Neoplasms may be benign or malignant. The distinction between benign and malignant tumors is less evident in the CNS than in other organs. E.g. histologically benign tumors may infiltrate large regions of the brain or may have high morbidity and mortality because of their location in the nervous system. The ability to resect CNS tumors depends on location, and may be limited.