Pilocytic astrocytoma

Pilocytic astrocytoma
Diagnosis in short
Pilocytic astrocytoma. Smear. H&E stain.
LM DDx	piloid gliosis, oligodendroglioma, glioblastoma
Stains	PAS-D +ve (eosinophilic granular bodies)
IHC	GFAP +ve
Gross	usually cerebellar +/-cystic
Site	brain - usu. cerebellum
Prevalence	common - esp. in children
Prognosis	good (WHO Grade I)

Pilocytic astrocytoma, abbreviated PA, is a low-grade astrocytoma. It the most common glioma in children.

General

• Low-grade astrocytoma - WHO Grade I by definition, but rare anaplastic forms have been described.
• Classically in the cerebellum in children; most common glioma in children.
• The optic glioma is associated with neurofibromatosis 1.
• Rare variants include Pilomyxoid astrocytoma and Anaplastic pilocytic astrocytoma.

Imaging

• Well-defined, T2-hyperintense.
• Strong CM enhancement.
• May contain cysts.
• Associated with midline structures.

Gross

Features:

• Usually well-circumscribed, soft.
• Can be cystic with mural nodule.
• Optic gliomas may present as fusiform mass.
• Occ. calcium deposits and hemosiderin.

Microscopic

Features:

• Classically biphasic (though either may be absent):
  1. Fibrillar.
  2. Microcystic/loose.
• Hair-like fibres ~ 1 micrometer; pilo- = hair.
  • Best seen on smear or with GFAP IHC.
• Rosenthal fibres - key feature.
  • May be rare. Not pathognomonic (see below).
• Eosinophilic granular bodies.
• Low cellularity - when compared to medulloblastoma and ependymoma.

Notes:

• +/-Microvascular proliferation.
• +/-Focal necrosis.
  • Necrosis with pseudopalisading more likely glioblastoma.
• +/-Mitoses - not significant in the context of the Dx.

DDx (of Rosenthal fibers):

• Chronic reactive gliosis.
• Subependymoma.
• Ganglioglioma.
• Alexander's disease (rare leukodystrophy).

DDx of pilocystic astrocytoma (brief):

• Piloid gliosis (esp. in sellar lesions).
• Oligodendroglioma.
• Glioblastoma (uncommon - but important).
• Tanycytic Ependymoma
• Pilocytic tumor components may be found in Ganglioglioma, DNET, RGNT

Images

Smears

• 

  Bipolar cells with hair-like processes - smear - very high mag. (WC)

• 

  EGBs - smear. (WC/AFIP)

• 

  Rosenthal fibres - smear. (WC/AFIP)

Sections

• 

  Rosenthal fibres. (WC)

• 

  Occasional pleomorphism. (WC)

• 

  Microvascular proliferation. (WC)

www:

• Rosenthal fibre (ouhsc.edu).
• Pilocytic astrocytoma (upmc.edu).
• Pilocytic astrocytoma - another case (upmc.edu).
• Pilocytic astrocytoma - pennies on a plate (upmc.edu).
• Pilocytic astrocytoma (upmc.edu).

Stains

• PAS-D: eosinophilic granular bodies +ve.

IHC

Features:

• GFAP +ve (fibres).
• CD68: may have a significant macrophage component.
• KI-67: may be "high" (~20% ???).
• Olig 2: Usually strongly present.
• IDH1 (R132H) -ve.
• H3F3A (K27M) -ve.

Molecular

• Almost all alteration associated with the MAPK pathway.
• KIAA1549-BRAF fusion transcripts most common in sporadic PA (up to 2/3 of all cases).
  • DDx: Fusion reported in rare Diffuse Leptomeingeal Glioneuronal Tumors and Oligodendroglioma.
• Rarely BRAF, KRAS or FGFR1 mutations, NTRK2, SRGAP3-RAF1 or FAM131B-BRAF fusions.
• Up to 15% of all NF1 patients develop a PA ("optic glioma" as predilection).
• Rare reports of PA in Noonan-Syndrome (PTPN11 mutation).

Prognosis

• Excellent (10-year OS: 90%)
• In thalamic/chiasmatic region not so good (incomplete resection, often Pilomyxoid astrocytoma).
• Primary treatment: surgery. Incomplete resection: RT has to be considered.
  • Chx is given in rare cases that are still progredient

See also

• Neuropathology tumours.
• Astrocytoma.