Note: Some citations in the text of this section are followed by a level of evidence. The PDQ editoiral boards use a formal ranking system to help the reader judge the strength of evidence linked to the reported results of a therapeutic strategy. (Refer to the PDQ summary on Levels of Evidence for more information.)

This classification is based on the World Health Organization (WHO) classification of nervous system tumors.[1] The WHO approach incorporates and interrelates morphology, cytogenetics, molecular genetics, and immunologic markers in an attempt to construct a cellular classification that is universally applicable and prognostically valid. Earlier attempts to develop a TNM-based classification were dropped: tumor size (T) is less relevant than tumor histology and location, nodal status (N) does not apply because the brain and spinal cord have no lymphatics, and metastatic spread (M) rarely applies because most patients with central nervous system (CNS) neoplasms do not live long enough to develop metastatic disease.[2]

The WHO grading of CNS tumors establishes a malignancy scale based on histologic features of the tumor.[3] The histologic grades are as follows:

WHO GRADE I includes lesions with low proliferative potential, a frequently discrete nature, and the possibility of cure following surgical resection alone.

WHO GRADE II includes lesions that are generally infiltrating and low in mitotic activity but recur. Some tumor types tend to progress to higher grades of malignancy.

WHO GRADE III includes lesions with histologic evidence of malignancy, generally in the form of mitotic activity, clearly expressed infiltrative capabilities, and anaplasia.

WHO GRADE IV includes lesions that are mitotically active, necrosis-prone, and generally associated with a rapid preoperative and postoperative evolution of disease.

The following outline has been adapted from the WHO classification. Tumors of glial origin are grouped under a common heading, and tumors limited to the peripheral nervous system have been excluded. Some rare or exclusively pediatric tumors are listed below for purposes of classification, but they are not discussed in the text that follows.

1. Neuroepithelial tumors.
   1. Glial tumors.
      1. Astrocytic tumors.
         1. Pilocytic astrocytoma.
         2. Diffuse astrocytoma (including fibrillary, protoplasmic, and gemistocytic).
         3. Anaplastic astrocytoma.
         4. Glioblastoma (including giant cell glioblastoma, and gliosarcoma).
         5. Pleomorphic xanthoastrocytoma.
         6. Subependymal giant cell astrocytoma.
      2. Oligodendroglial tumors.
         1. Oligodendroglioma.
         2. Anaplastic oligodendroglioma.
      3. Mixed gliomas.
         1. Oligoastrocytoma.
         2. Anaplastic oligoastrocytoma.
      4. Ependymal tumors.
         1. Myxopapillary ependymoma.
         2. Subependymoma.
         3. Ependymoma (including cellular, papillary, clear cell, and tanycytic).
         4. Anaplastic ependymoma.
      5. Neuroepithelial tumors of uncertain origin.
         1. Astroblastoma.
         2. Chordoid glioma of the third ventricle.
         3. Gliomatosis cerebri.
   2. Neuronal and mixed neuronal-glial tumors (some glial component may be present).
      1. Gangliocytoma.
      2. Ganglioglioma.
      3. Desmoplastic infantile astrocytoma/ganglioglioma.
      4. Dysembryoplastic neuroepithelial tumor.
      5. Central neurocytoma.
      6. Cerebellar liponeurocytoma.
      7. Paraganglioma.
   3. Nonglial tumors.
      1. Embryonal tumors.
         1. Ependymoblastoma.
         2. Medulloblastoma.
         3. Supratentorial primitive neuroectodermal tumor (PNET).
      2. Choroid plexus tumors.
         1. Choroid plexus papilloma.
         2. Choroid plexus carcinoma.
      3. Pineal parenchymal tumors.
         1. Pineoblastoma.
         2. Pineocytoma.
         3. Pineal parenchymal tumor of intermediate differentiation.
2. Meningeal tumors.
   1. Meningioma.
   2. Hemangiopericytoma.
   3. Melanocytic lesion.
3. Germ cell tumors.
   1. Germinoma.
   2. Embryonal carcinoma.
   3. Yolk-sac tumor (endodermal-sinus tumor).
   4. Choriocarcinoma.
   5. Teratoma.
   6. Mixed germ cell tumor.
4. Tumors of the sellar region.
   1. Pituitary adenoma. (Refer to the PDQ summary on Pituitary Tumor Treatment for more information.)
   2. Pituitary carcinoma.
   3. Craniopharyngioma.
5. Tumors of uncertain histogenesis.
   1. Capillary hemangioblastoma.
6. Primary CNS lymphoma. (Refer to the PDQ summary on Primary CNS Lymphoma Treatment for more information.)
7. Tumors of peripheral nerves that affect the CNS.
   1. Schwannoma.
8. Metastatic tumors.