Appendix E. The new WHO Classification of Tumours affecting the Central Nervous System

Appendix E. 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 III or even IV rather than grade II 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.

Neuroepithelial Tumors of the CNS (first five main types)

I. Astrocytic tumours [glial tumours — categories I–V, below — 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 II)

a. variants: protoplasmic, gemistocytic, fibrillary, mixed

2. Anaplastic (malignant) astrocytoma (WHO grade III)

a. hemispheric

b. diencephalic

c. optic

d. brain stem

e. cerebellar

3. Glioblastoma multiforme (WHO grade IV)

a. variants: giant cell glioblastoma, gliosarcoma

4. Pilocytic astrocytoma [non‑invasive, WHO grade I]

a. hemispheric

b. diencephalic

c. optic

d. brain stem

e. cerebellar

5. Subependymal giant cell astrocytoma [non‑invasive, WHO grade I]

6. Pleomorphic xanthoastrocytoma [non‑invasive, WHO grade I]

II. Oligodendroglial tumors

1. Oligodendroglioma (WHO grade II)

2. Anaplastic (malignant) oligodendroglioma (WHO grade III)

III. Ependymal cell tumours

1. Ependymoma (WHO grade II)

a. variants: cellular, papillary, epithelial, clear cell, mixed

2. Anaplastic ependymoma (WHO grade III)

3. Myxopapillary ependymoma

4. Subependymoma (WHO grade I)

IV. Mixed gliomas

1. Mixed oligoastrocytoma (WHO grade II)

2. Anaplastic (malignant) oligoastrocytoma (WHO grade III)

3. Others (e.g. ependymo‑astrocytomas)

V. Neuroepithelial tumours of uncertain origin

1. Polar spongioblastoma (WHO grade IV)

2. Astroblastoma (WHO grade IV)

3. Gliomatosis cerebri (WHO grade IV)

A number of grading systems are in common use for tumours of astrocytic lineage (i.e. 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.

Grading of astrocytic tumours

WHO designation

WHO grage*

Kernohan grade*

St. Anne/Mayo grade

St. Anne/Mayo criteria

pilocytic astrocytoma

I

I

Excluded

astrocytoma

II

I, II

1

No criteria fulfilled

2

one criterion: usually nuclear atypia

Anaplastic (malignant) astrocytoma

III

II, III

3

two criteria: usually nuclear atypia and mitosis

glioblastoma

IV

III, IV

4

three or four criteria: usually the above and necrosis and/or endothelial proliferation

* The WHO and Kernohan systems are not criteria based. Thus, a given tumour may not fall under the same designation in all three systems.