Glioma

A glioblastoma, the most common glioma. (WC/sbrandner)

Gliomas are the most compon primary tumor in the brain and spinal cord. They originate from glial cells and their precursors.

Classification

Gliomas are classified by cell type, by CNS WHO grade, patient age and by location.

By type of cell

Gliomas are named according to the specific type of cell with which they share histological features, but not necessarily from which they originate. The main types of gliomas are:

Glioblastoma is a malignant astrocytoma and the most common primary brain tumor among adults.
Astrocytomas—astrocytes.
Oligodendrogliomas—oligodendrocytes.
Ependymomas—ependymal cells.
Glioneuronal and neuronal tumors.

Note: Some authors consider choroid plexus as a specific type of glia, but choroid plexus tumors are usually not covered under the umberella term glioma.

By CNS WHO grade

Gliomas are further categorized according to their grade, which is determined by pathologic evaluation of the tumor. Of numerous grading systems in use, the most common is the World Health Organization (WHO) grading system under which tumors are graded from 1 (least advanced disease—best prognosis) to 4 (most advanced disease—worst prognosis). Historically roman numerals were used for grading, but have been replaced by arabic numerals for grading within tumor types. Specific molecular features may influence tumor grading which is now a combination of histology and molecular neuroathology. Grading is based on expected biological behaviour, although proper treatment may heavily change prognosis.

Low-grade gliomas [CNS WHO grade 1-2] are well-differentiated (not anaplastic); these tend to exhibit benign tendencies and portend a better prognosis for the patient. However, they have a uniform rate of recurrence and some diffuse gliomas may increase in grade over time and therefore calling these tumours benign is avoided.
High-grade [CNS WHO grade 3-4] gliomas are undifferentiated or anaplastic; these are malignant and carry a worse prognosis.

By patient age group

The fifth CNS WHO edition recognizes following categories:

Adult-type diffuse gliomas.
Pediatric-type diffuse low-grade gliomas.
Pediatric-type diffuse high-grade gliomas.
Circumscribed astrocytic gliomas.

By location

A supratentorial tumour is above the tentorium, in the cerebrum, and mostly found in adults (70%).
An infratentorial tumour is below the tentorium, in the cerebellum, and mostly found in children (70%).
The pontine is located in the pons of the brainstem. The brainstem has three parts (pons, midbrain, and medulla); the pons controls critical functions such as breathing, making surgery on these extremely dangerous.

Anatomic separation is essential in ependymal tumors. They are classfied into three groups:
- Supratentorial ependymoma.
- Posterior fossa ependymoma.
- Spinal ependymoma.

Some gliomas have a specific anatomic designation in their name, reflecting their predominant tumor location.
- Diffuse hemispheric glioma, H3 G34-mutant.
- Diffuse midline glioma, H3 K27-altered.
- Infant-type hemispheric glioma.

By Molecular profile

Some tumors can only diagnosed after a specific alteration has been confirmed.

Examples are:

Astrocytoma, IDH-mutant.
Oligodendroglioma, IDH-mutant and 1p/19q codeleted.
Diffuse hemispheric glioma, H3 G34-mutant.
Diffuse midline glioma, H3 K27-altered.
Diffuse astrocytoma, MYB- or MYBL-altered.
CNS tumour with BCOR internal tandem duplication.

Table of common gliomas

Histomorphologic comparison of common gliomas:

Entity	Rosenthal fibres / EGBs	Nuclear atypia	Mitoses	Necrosis	MVP	Infiltrative	Image
Pilocytic astrocytoma	yes	usu. no	usu. no	usu. no	yes	no
Astrocytoma CNS WHO grade 2	no	yes	no	no	no	yes
Astrocytoma CNS WHO grade 3	no	yes	yes	no	no	yes
Glioblastoma	no	yes	yes	yes	yes	yes
Oligodendroglioma	no	usu. no	yes	no	no	yes
Ependymoma CNS WHO grade 2	no	usu. no	usu. no	usu. no	no	discrete
Ependymoma CNS WHO grade 3	no	yes	yes	usu. yes	rare	discrete