WHO CNS Tumor Classification - A PG Pathology Study Guide

Why the 2021 (5th Edition) Matters

The 3 Big Families You Must Know

1. GLIOMAS (most common primary intraparenchymal brain tumors)

A. Adult-type Diffuse Gliomas (3 types - the most tested group)

• Glioblastoma grossly: butterfly lesion crossing corpus callosum; histology - pseudopalisading necrosis + microvascular proliferation are the two hallmarks
• Glioblastoma = ~14% of all primary CNS tumors, >50% of CNS malignancies
• Even without necrosis/microvascular proliferation, an IDH-wildtype diffuse astrocytoma qualifies as GBM if it has: TERT promoter mutation OR EGFR amplification OR +7/-10

B. Pediatric-type Diffuse Gliomas

• Diffuse midline glioma, H3 K27-altered (e.g., DIPG - diffuse intrinsic pontine glioma) - Grade 4
• Diffuse hemispheric glioma, H3 G34-mutant - Grade 4
• Diffuse low-grade glioma, MAPK pathway-altered - Grade 1/2

C. Circumscribed Astrocytic Gliomas

• Pilocytic astrocytoma (Grade 1): Most common in children; KIAA1549::BRAF fusion; cerebellar, cystic with mural nodule; biphasic pattern, Rosenthal fibers, eosinophilic granular bodies; microvascular proliferation/necrosis do NOT upgrade it
• Pleomorphic xanthoastrocytoma (Grade 2-3): BRAF V600E mutation

2. EPENDYMOMAS

3. EMBRYONAL TUMORS

1. WNT-activated - best prognosis, monosomy 6, nuclear beta-catenin
2. SHH-activated - intermediate, TP53 wild-type or mutant (important distinction)
3. Group 3 - worst prognosis; MYC amplification, metastatic
4. Group 4 - most common; isochromosome 17q

Other Important Tumor Categories

Meningiomas (most common primary brain tumor overall)

• Grade 1 (benign, most), Grade 2 (atypical), Grade 3 (malignant/rhabdoid/papillary)
• Key IHC: EMA+, S100-
• NF2 mutation common; brain invasion alone now = Grade 2

Nerve Sheath Tumors

• Schwannoma: S100+; bilateral acoustic = NF2; Antoni A and B areas; Verocay bodies
• Neurofibroma: S100 partially+; NF1 associated

Lymphomas

• Primary CNS DLBCL: most common primary CNS lymphoma; periventricular, CD20+, EBV-negative in immunocompetent; "angel wing" enhancement on MRI

Metastases

• Most common tumors involving the CNS overall (not primary)
• Common sources: lung (most common), breast, melanoma, GI, renal

The 5-Step Framework to Answer Any WHO CNS Classification MCQ

1. Is it diffuse or circumscribed?
   • Diffuse → think adult (IDH/1p19q) or pediatric (H3/MAPK)
   • Circumscribed → pilocytic, ependymoma, etc.
2. What is the IDH status?
   • IDH mutant → astrocytoma OR oligodendroglioma
   • IDH wildtype → GBM (grade 4)
3. If IDH mutant, check 1p/19q:
   • Codeleted → Oligodendroglioma
   • Not codeleted → Astrocytoma, IDH-mutant
4. Grade within the type:
   • Astrocytoma IDH-mutant: CDKN2A biallelic deletion = automatic grade 4
   • GBM IDH-wildtype: always grade 4
5. Key IHC panels:
   • GFAP = glial tumors
   • IDH1 R132H antibody = 90% of IDH mutations
   • ATRX loss = astrocytoma (not oligodendroglioma)
   • p53 overexpression = IDH-mutant astrocytoma
   • OLIG2 = oligodendroglial lineage
   • Synaptophysin, NeuN = neuronal tumors
   • INI1/SMARCB1 loss = AT/RT
   • SSTR2 = meningioma (useful for PET)

High-Yield Points for MCQs / DNB / USMLE

• "Pseudopalisading necrosis" = GBM, IDH-wildtype
• "Fried egg" cells with perinuclear halo + chicken-wire vasculature = Oligodendroglioma
• "Biphasic + Rosenthal fibers + Eosinophilic granular bodies + mural nodule in cyst" = Pilocytic astrocytoma
• "Rosettes (Homer-Wright)" = Medulloblastoma / neuroblastoma
• "Flexner-Wintersteiner rosettes" = Retinoblastoma
• "True ependymal rosettes + perivascular pseudorosettes" = Ependymoma
• "SMARCB1/INI1 loss in a child <3 years" = AT/RT
• "IDH-mutant grade 4 astrocytoma" ≠ GBM (GBM is reserved for IDH-wildtype only)
• Glioblastoma crosses corpus callosum = "butterfly glioma"
• BRAF V600E = pleomorphic xanthoastrocytoma (also papillary craniopharyngioma)
• BRAF-KIAA1549 fusion = pilocytic astrocytoma

Sources

• Robbins and Cotran Pathologic Basis of Disease (10th ed.), Chapter 28 - CNS Tumors, pp. 1193-1230
• Pathology Outlines - WHO CNS Classification
• Louis DN et al., The 2021 WHO Classification of CNS Tumors - Acta Neuropathol 2021

Basic Cell Cytology of the Brain and the Origin of CNS Tumors

The Two Major Cell Populations of the CNS

1. Neurons - the functional unit; signal processing
2. Glia (Neuroglia) - support cells; ~10x more numerous than neurons in some regions

1. NEURONS

Cytology

• Large cells with a prominent round nucleus and a large, conspicuous nucleolus ("owl-eye nucleus" in context)
• Abundant Nissl substance (rough ER + free ribosomes) = basophilic granules in cytoplasm - reflects high protein synthesis demand
• Cytoplasmic processes: dendrites (receive input) and a single axon (transmit output)
• Completely postmitotic - cannot divide once mature; destroyed neurons are not replaced

Key IHC markers: Synaptophysin, NeuN, Neurofilament protein, Chromogranin

Response to injury

• Red neuron (acute ischemia): cell body shrinks, nucleus becomes pyknotic, cytoplasm intensely eosinophilic, Nissl substance disappears - visible by 6-12 hours
• Central chromatolysis (axonal injury/regeneration): cell body swells and rounds, nucleus pushed to periphery, Nissl substance disperses from center
• Neuronophagia: microglia surround and engulf dying neurons

Tumor origin

• Ganglioglioma / Gangliocytoma - mature ganglion cells + neoplastic glial component
• Dysembryoplastic neuroepithelial tumor (DNET) - young patients, temporal lobe, seizures
• Medulloblastoma - embryonal tumor from cerebellar granule cell progenitors

2. ASTROCYTES

Cytology

• Star-shaped (Greek: astron = star) with multiple radiating cytoplasmic processes
• Nucleus: oval, pale chromatin, inconspicuous nucleolus
• Cytoplasm typically indistinct on H&E; identified by IHC
• Two subtypes: fibrous astrocytes (white matter - long slender processes) and protoplasmic astrocytes (gray matter - short bushy processes)
• Hallmark intermediate filament: GFAP (Glial Fibrillary Acidic Protein)

Key IHC marker: GFAP+, S100+, Vimentin+

Functions

• Metabolic buffering: regulate K+, neurotransmitter reuptake (glutamate)
• Blood-brain barrier: astrocyte foot processes envelop capillary endothelium
• Structural scaffold and repair via gliosis
• Radial glia during development = a specialized astrocyte that acts as a scaffold for neuronal migration

Response to injury - GLIOSIS

• Most important CNS injury marker regardless of etiology
• Astrocytes undergo hypertrophy + hyperplasia → gemistocytic (reactive) astrocytes: plump, bright pink cytoplasm, eccentric nucleus, stout processes (GFAP overexpressed)
• Alzheimer type II astrocytes: seen in hyperammonemia (liver disease/urea cycle defects) - enlarged vesicular nucleus, glycogen droplet
• Rosenthal fibers: thick eosinophilic corkscrew inclusions in astrocyte processes; contain αB-crystallin, HSP27, ubiquitin; seen in long-standing gliosis and pilocytic astrocytoma
• Corpora amylacea: PAS-positive, basophilic, lamellated round bodies at subpial/perivascular astrocyte end-feet; increase with age (degenerative)

Reactive astrocytes (gemistocytes). GFAP immunostain (brown) highlights the star-shaped cytoplasmic processes. From Robbins Pathologic Basis of Disease.

Tumor origin from astrocytes

3. OLIGODENDROCYTES

Cytology

• Small, round nucleus with densely packed chromatin (dark, "raisin-like")
• Scanty cytoplasm - barely visible on H&E
• Located in rows between nerve fibers in white matter (interfascicular oligodendrocytes) or scattered in gray matter (satellite oligodendrocytes)
• Each oligodendrocyte wraps around multiple internodes on multiple axons (contrast: Schwann cells in PNS wrap only one internode on one axon)

Key IHC markers: OLIG2+ (most sensitive), MBP (myelin basic protein), MOG

Function

• Myelination of CNS axons → enables saltatory conduction
• Satellite oligodendrocytes may play a metabolic support role for neurons

Response to injury

• Injury → demyelination (as in MS, PML, leukodystrophies)
• In PML (JC virus infection): viral inclusions appear within oligodendroglial nuclei
• In MSA (Multiple System Atrophy): glial cytoplasmic inclusions (GCIs) composed of α-synuclein accumulate in oligodendrocytes

Tumor origin

• Oligodendroglioma, IDH-mutant and 1p/19q codeleted (Grade 2 or 3)
• Classic histology: "fried egg" cells (perinuclear halo artifact) + "chicken-wire" vasculature + microcalcifications
• The "fried egg" perinuclear halo is a fixation artifact - not seen in frozen sections
• OLIG2 is positive in both oligodendrogliomas and astrocytomas, so it alone does not distinguish them - molecular testing is essential

4. MICROGLIA

Cytology

• Smallest glial cells
• Elongated, bean-shaped / irregular nucleus with dense chromatin
• Thin, ramified (branching) cytoplasmic processes with spine-like appendages
• Appear inconspicuous on H&E; identified by IHC

Key IHC markers: CD68+, IBA-1+, CD163+ (shared with macrophages)

Origin - UNIQUE among CNS cells

• NOT from neuroectoderm - derived from yolk sac (or fetal liver) early in embryonic development - mesodermal origin
• They are the resident macrophages of the CNS
• Share surface markers with peripheral monocytes/macrophages

Functions

• Immune surveillance - patrol CNS for pathogens and cell debris
• Synaptic pruning during development (complement-dependent) - elimination of weak synapses
• Phagocytosis of dead cells, debris, pathogens

Response to injury

1. Proliferation
2. Nuclear elongation → "rod cells"
3. Microglial nodules - aggregates around small foci of necrosis (seen in viral encephalitis)
4. Neuronophagia - microglia surround dying neurons

Tumor origin

5. EPENDYMAL CELLS

Cytology

• Ciliated, columnar epithelial-like cells lining the ventricular system and central canal of spinal cord
• Cilia beat to circulate CSF
• Cuboidal to columnar in cross-section; form a single layer
• Specialized subtype: tanycytes (in floor of 3rd ventricle); choroid plexus epithelium (produces CSF)

Key IHC markers: GFAP+ (variable), EMA+, Vimentin+, S100+

Function

• Line and protect ventricular walls
• CSF circulation (ciliary movement)
• Contribute to blood-CSF barrier

Response to injury

• Ependymal granulations: when ependymal lining disrupted by inflammation or ventricular dilation, subependymal astrocytes proliferate, forming small nodular irregularities on ventricular surface
• CMV can infect ependymal cells → nuclear inclusions

Tumor origin

• Ependymomas arise from ependymal cells or their radial glial precursors
• Location + molecular alteration now defines them (WHO 2021):
  • Supratentorial: ZFTA::RELA fusion (poor prognosis) or YAP1 fusion (better)
  • Posterior fossa: PFA (H3K27me3 loss, poor) vs PFB (better)
  • Spinal: NF2 mutation (most common spinal)
• Histology: perivascular pseudorosettes (most common - tumor cells around vessel with anuclear zone) + true ependymal rosettes (cells around a central lumen)

6. SUMMARY TABLE: Cell → Tumor

The Key Concept: Tumors from Progenitor Cells, Not Mature Cells

"It appears these tumors are mostly derived from multipotent progenitor cells that preferentially differentiate down a particular cellular lineage."

• A "glioblastoma" expresses GFAP (astrocytic) but also has oligodendroglial features in some areas
• Medulloblastoma (embryonal tumor) can express neuronal AND glial markers
• The "cell of origin" and the "differentiation lineage" are not always the same thing

Quick Memory Aid for PG Exams

• Neurons → postmitotic; red neuron = ischemia; Nissl = RER
• Astrocytes → GFAP is the marker; gliosis = injury marker; gemistocytes = reactive
• Oligodendrocytes → myelinate CNS; OLIG2; "fried egg" = oligodendroglioma
• Microglia → yolk sac origin; CD68; rod cells + microglial nodules = viral encephalitis
• Ependymal → ciliated, ventricular lining; perivascular pseudorosettes = ependymoma
• Rosenthal fibers = astrocyte inclusions in pilocytic astrocytoma + long-standing gliosis
• Corpora amylacea = aging astrocytes; PAS+, basophilic, perivascular