Astrocytoma, IDH-mutant

Astrocytoma, IDH-mutant is a diffusely-growing, infiltrating astrocytoma of the adult occurring in the CNS white matter. IDH1 codon 132 or IDH2 codon 172 mutations is mandatory for diagnosis and are frequently associated with ATRX and TP53 mutations.

General

The current WHO classification recognizes three tumour grades:

• Astrocytoma, IDH mutant grade 2 (ICD-O: 9400/3)
• Astrocytoma, IDH mutant grade 3 (ICD-O: 9401/3)
• Astrocytoma, IDH mutant grade 4 (ICD-O: 9445/3)
• Diffuse astrocytoma,NOS (ICD-O 9400/3) - genetic testing still missing.

Astrocytoma, IDH mutant grade 2

• Most common CNS grade 2 WHO glioma in adults (peaks between 30-40 years).
• 10-15% of all astrocytomas.
• Usually shows progression to WHO CNS grade 3 sooner or later.

Astrocytoma, IDH mutant grade 3

• Most common CNS grade 3 WHO glioma in adults (peaks between 40-50 years).
• Approx 5% of all gliomas.
• Usually shows progression to WHO CNS grade 4 sooner or later.
• Overall prognosis is rather poor (average survival 2-3 years).
• Grade 3 tumors share a similiar prognosis to grade 2 IDH-mutant tumors.

Astrocytoma, IDH mutant grade 4

• Formely called glioblastoma, IDH-mutant or "secondary glioblastoma".
• CDKN2A deletion in grade 2 or grade 3 tumors results in upgrading to CNS WHO grade 4.

Radiology/Clinic

• Mass effect.
• Seizures.
• Neurologic decifit.
• CNS grade 2: Usually not contrast-enhanching, T2 bright.
• CNS grade 3 and 4: The majority are contrast-enhanching, T2 bright.

Macroscopy

• No clear demarcation from white matter.
• Softer consistency and opacity.
• May contain larger cysts
• Invaded structures may appear enlarged.
• CNS grade 2 and 3: No necrosis.

Histology

CNS grade 2 features:

• Cell density higher than normal brain.
• Mild to moderate nuclear pleomorphism.
  • Monotony of atypical nuclei and irregular distribution indicates neoplasm.
  • "naked nuclei" without recognizeable processes.
  • No prominent nucleolus.
• Cytoplasm highly variable (even within the same tumour).
  • In normal CNS the cytoplasm blends within the neuropil.
• Mitoses absent or very rare.
• Microcystic spaces of the background (none to extensive).
• Lymphocytic cuffing (mostly in gemistocytic type)
• Abent to few rosenthal fibers.

CNS grade 3 features:

• Increased cellularity (compared to CNS grade 2).
• Mitoses present (a single mitosis in a small specimen indicates a high-grade tumor).
  • Specimens with low cellularity but plenty of mitoses are also considered grade 3.
• Distinct nuclear atypia and pleomorphism.
  • May include multinucleated cells.
• Cytoplasm highly variable (even within the same tumour).
• Microcystic spaces of the background (none to extensive).
• No necrosis, no vascular proliferations.
  • Except radiation necrosis after pretreatment.

CNS grade 4 features:

• Increased cellularity (compared to CNS grade 2).
• Mitoses frequently present.
• Distinct nuclear atypia and pleomorphism.
• Multinucleated cells.
• Microvascular proliferation.
• Necrosis (less common than in glioblastoma).

IHC

• GFAP+ve.
• MAP2+ve (especially in cell processes).
• OLIG2 +ve.
• Vimentin+ve (often perinuclear).
• S-100+ve.
• p53: Nuclear staining in 30% of the tumours (usually few cells).
• MIB-1: CNS grade 2: 0-5% (mean: 2%); CNS grade 3 usu. 5-10%.
• IDH-1 (R132H)+ve in 60-70%.
  • 'Note: The mutation-specific antibody does not detect other less common IDH1/2 hotspot mutations.
• ATRX nuclear loss.

Molecular

• IDH1 R132- or IDH2 R172-hotspot mutations are mandatory.
• Absence of LOH 1p/19q (otherwise classify tumor as oligodendroglioma, IDH-mutant and 1p/19q codeleted).
• Tp53 mutations in approx. 60% (80-90% in gemistocytic, 50% in fibrillary types).
• MGMT promotor methylated in approx. 50%.
• CDKN2A/B homozygous deletion results in CNS grade 4

Note:

• The existence of diffuse astrocytoma, IDH wildtype is challenged.
  • Most adult cases show genetic alterations compatible with glioblastoma.
  • Molecular upgrade according to cIMPACT-NOW Update 3 consensus (one of these is sufficient):
    • EGFR amplification
    • Combined whole chromosome 7 gain and whole chromosome 10 loss (+ 7/− 10)
    • TERT promoter mutation
  • Suggested Sign-out:

     Diffuse astrocytic glioma, IDH-wildtype, with molecular features of glioblastoma, WHO grade IV

  • WHO grade II diffuse gliomas IDH-wt/H3-wt in children and adolescents have an indolent clinical behavior and rare anaplastic progression.
    • Most tumors show a BRAFV600E mutation, an FGFR alteration, or a MYB or MYBL1 rearrangement.
    • Glial morphology can be astrocytic or oligodendrocytic.

Molecular

• TERT promotor mutations in 20-25%
• Approximately 80 % of IDH wildtype astrocytomas in fact represent underdiagnosed GBM.

• 

  GFAP in astrocytoma (WC/jensflorian)

• 

  ATRX loss in astrocytoma (WC/jensflorian)

• 

  Diffuse astrocytoma, H&E (WC/jensflorian)

• 

  Diffuse astrocytoma, H&E (WC/jensflorian)

• 

  Astrocytoma, fibrillary type (WC/jensflorian)

• 

  Astrocytoma, protoplasmatic type (WC/jensflorian)

• 

  Gemistocytic astrocytoma (WC/jensflorian)

• 

  Marked mitotic activity in anaplastic astrocytoma (WC/jensflorian).

• 

  Marked nuclear pleomorphism (AFIP).

DDx

For CNS grade 2 tumours:

• Reactive astrocytosis.
• Demyelinisation.
• Grade 3 astrocytoma, IDH mutant - increased mitotic activity.
• Oligodendroglioma, IDH-mutant and 1p/19q codeleted - esp. protoplasmatic forms. LOH 1p/19q testing required.
• SEGA - esp. gemistocytic forms.
• Diffuse glioma, MYB- or MYBL1-altered.

For CNS grade 3 tumours:

• Anaplastic Oligodendroglioma, when LOH 1p/19q is present.

For CNS grade 4 tumours:

• Glioblastoma - vascular proliferations and / or necrosis.

Outdated terminologies

• Diffuse astrocytoma
• Gemistocytic astrocytoma
• Diffuse astrocytoma, IDH-wildtype
• Glioblastoma; IDH-mutant
• Fibrillary astrocytoma (ICD-O: 9420/3)
• Protoplasmatic astrocytoma (ICD-O:9410/3)

See also

• Astrocytoma.
• Neuropathology_tumours#Infiltrative_astrocytomas