11 The new WHO classification of tumours affecting the central nervous system
In 1993, the WHO ratified a new comprehensive classification of neoplasms affecting the central nervous system. The classification of brain tumours is based on the premise that each type of tumour results from the abnormal growth of a specific cell type. To the extent that the behaviour of a tumour correlates with basic cell type, tumour classification dictates the choice of therapy and predicts prognosis. The new WHO system is particularly useful in this regard with only a few notable exceptions (for example all or almost all gemistocytic astrocytomas are actually anaplastic and hence grade 3 or even 4 rather than grade 2 as designated by the WHO system). The WHO classification also provides a parallel grading system for each type of tumour. In this grading system most named tumours are of a single defined grade. The new WHO classification provides the standard for communication between different centres around the world. An outline of this classification is provided below.
11.1 Neuroepithelial tumours of the CNS (first 5 main types)
1. Astrocytic tumours (glial tumours categories 1 to 5 may also be subclassified as invasive or non‑invasive, although this is not formally part of the WHO system, the non‑invasive tumour types are indicated below. Categories in italics are also not recognized by the new WHO classification system, but are in common use.)
1. Astrocytoma (WHO grade 2)
a. variants: protoplasmic, gemistocytic, fibrillary, mixed
2. Anaplastic (malignant) astrocytoma (WHO grade 3)
a. hemispheric
b. diencephalic
c. optic
d. brain stem
e. cerebellar
3. Glioblastoma multiforme (WHO grade 4)
a. variants: giant cell glioblastoma, gliosarcoma
4. Pilocytic astrocytoma 9non‑invasive, WHO grade 1)
a. hemispheric
b. diencephalic
c. optic
d. brain stem
e. cerebellar
5. Subependymal giant cell astrocytoma (non‑invasive, WHO grade 1)
6. Pleomorphic xanthoastrocytoma (non‑invasive, WHO grade 1)
2. Oligodendroglial tumours
1. Oligodendroglioma (WHO grade 2)
2. Anaplastic (malignant) oligodendroglioma (WHO grade 3)
3. Ependymal cell tumours
1. Ependymoma (WHO grade 2)
a. variants: cellular, papillary, epithelial, clear cell, mixed
2. Anaplastic ependymoma (WHO grade 3)
3. Myxopapillary ependymoma
4. Subependymoma (WHO grade 1)
4. Mixed gliomas
1. Mixed oligoastrocytoma (WHO grade 2)
2. Anaplastic (malignant) oligoastrocytoma (WHO grade 3)
3. Others (for example, ependymo‑astrocytomas)
5. Neuroepithelial tumours of uncertain origin
1. Polar spongioblastoma (WHO grade 4)
2. Astroblastoma (WHO grade 4)
3. Gliomatosis cerebri (WHO grade 4)
A number of grading systems are in common use for tumours of astrocytic lineage (that is, astrocytomas, anaplastic astrocytomas and glioblastomas). Grades are assigned solely based on the microscopic appearance of the tumour. The numerical grade assigned for a given tumour, however, can vary depending on which grading system is used as illustrated by the following table. Thus, it is important to specify the grading system referred to when a grade is specified. The St. Anne-Mayo grade has proven to correlate better with survival than the previously common Kernohan grading system. It can only be applied to invasive tumours of astrocytic lineage; it is otherwise similar to the WHO grading system.
| WHO designation | WHO grade | Kernohan grade | St. Anne-Mayo grade | St. Anne-Mayo criteria |
|---|---|---|---|---|
|
pilocytic astrocytoma |
1 |
1 |
excluded |
‑ |
|
astrocytoma |
2 |
1, 2 |
1 |
no criteria fulfilled |
|
- |
- |
- |
2 |
1 criterion: usually nuclear atypia |
|
anaplastic (malignant) astrocytoma |
3 |
2, 3 |
3 |
2 criteria: usually nuclear atypia and mitosis |
|
glioblastoma |
4 |
3, 4 |
4 |
3 or 4 criteria: usually the above and necrosis and/or endothelial proliferation |
Note: The WHO and Kernohan systems are not criteria-based. Thus, a given tumour may not fall under the same designation in all 3 systems.