Circumscribed astrocytic gliomas are pediatric and young adult tumors that grow as well-defined, often cystic masses rather than infiltrating diffusely. The 2021 WHO classification recognizes them as a distinct group, separated from diffuse gliomas. The major entities — pilocytic astrocytoma, pleomorphic xanthoastrocytoma (PXA), subependymal giant cell astrocytoma (SEGA), and ganglioglioma — share favorable prognoses compared to diffuse gliomas, and many carry targetable molecular alterations. This page covers the major circumscribed gliomas and related neuronal-glial tumors.

Pilocytic Astrocytoma

Histology

  • Biphasic pattern: compact areas with bipolar (piloid, “hair-like”) cells alternating with loose microcystic areas.
  • Rosenthal fibers: brightly eosinophilic, irregular structures (aggregates of GFAP and αB-crystallin).
  • Eosinophilic granular bodies (EGBs): small eosinophilic globular structures.
  • Microvascular proliferation can be present (does not upgrade — these tumors are CNS WHO grade 1).
  • Microcystic change: extensive in some areas.

Molecular

  • KIAA1549-BRAF fusion: most common (especially cerebellar location).
  • BRAF V600E mutation: subset (especially supratentorial).
  • FGFR1, NF1 mutations: less common.
  • Almost always involves MAPK pathway activation.

Locations

  • Cerebellar hemispheres (most common; classic pediatric).
  • Optic pathway / hypothalamic (often in NF1).
  • Brainstem (often pilomyxoid variant — more aggressive).
  • Spinal cord.
  • Hemispheres.

Imaging

Classical cerebellar pilocytic astrocytoma: cystic with a mural nodule that enhances; cyst wall typically does not enhance.

Treatment and Prognosis

  • Complete resection is often curative.
  • Pilocytic astrocytoma is CNS WHO grade 1.
  • Excellent prognosis when surgically accessible.
  • Pilomyxoid variant (especially infants, hypothalamic): more aggressive, can recur.
  • MEK or BRAF inhibitors for inoperable / progressive cases.

Pleomorphic Xanthoastrocytoma (PXA)

Histology

  • Marked pleomorphism with bizarre giant cells.
  • Xanthomatous (lipid-laden) cells.
  • EGBs and Rosenthal fibers may be present.
  • Reticulin-rich (pericellular).
  • Anaplastic PXA (CNS WHO grade 3): ≥ 5 mitoses per 10 HPF; sometimes necrosis.

Molecular

  • BRAF V600E: ~70% of PXA.
  • CDKN2A/B homozygous deletion: common.

Clinical

  • Young patients (children, young adults).
  • Superficial cerebral hemispheres (temporal lobe common).
  • Often present with seizures.
  • Surgical resection often curative for low-grade.
  • BRAF inhibitors for V600E mutant tumors.

Subependymal Giant Cell Astrocytoma (SEGA)

Clinical

  • Almost always associated with tuberous sclerosis complex (TSC).
  • Arises at the foramen of Monro → obstructive hydrocephalus.
  • Children and young adults.

Histology

  • Large cells with abundant cytoplasm, large nuclei.
  • Astrocytic and neuronal features may coexist.
  • Often calcified.

Treatment

  • Resection if accessible.
  • mTOR inhibitors (everolimus, sirolimus): dramatically effective; can shrink tumors and prevent need for surgery. First-line for many SEGAs in TSC.

Ganglioglioma

Histology

  • Mixed neuronal and glial elements.
  • Dysplastic neurons (ganglion cells): clustered, abnormal arrangement.
  • Glial component is usually astrocytic.
  • EGBs may be present.
  • Synaptophysin and NeuN highlight the neuronal component.

Molecular

  • BRAF V600E: common.
  • Other MAPK pathway alterations.

Clinical

  • Children, young adults.
  • Temporal lobe common.
  • Often present with epilepsy.
  • Surgery curative for most.
  • Long-term epilepsy outcomes generally good.

Dysembryoplastic Neuroepithelial Tumor (DNET)

Histology

  • “Specific glioneuronal element”: multinodular pattern with bundles of axons traversing a mucinous matrix, with floating neurons (mature neurons within mucinous pools).
  • Oligodendrocyte-like cells.
  • Often cortically based.

Clinical

  • Children, young adults.
  • Temporal lobe common.
  • Drug-resistant epilepsy.
  • CNS WHO grade 1.
  • Resection often eliminates seizures.

Central Neurocytoma

  • Intraventricular (lateral ventricle) tumor in young adults.
  • Neurocytic differentiation (synaptophysin positive).
  • Often calcified.
  • Resection often curative.

Desmoplastic Infantile Ganglioglioma / Astrocytoma (DIG/DIA)

  • Infants under 2 years.
  • Large superficial cerebral tumor with prominent desmoplastic (collagen-rich) stroma.
  • Often dramatic at presentation but generally good outcomes after resection.

Papillary Glioneuronal Tumor / Rosette-Forming Glioneuronal Tumor

Rare; specific locations and histologic patterns; usually low-grade.

Subependymoma

  • Slowly growing intraventricular tumor in adults.
  • Often incidental.
  • Histology: clusters of cells in microcystic background; resemble ependymal cells.
  • CNS WHO grade 1.

Astroblastoma, MN1-Altered

  • Children, young adults.
  • Cerebral hemisphere.
  • Histology: perivascular pseudorosettes with broad foot processes (astroblastomatous).
  • MN1 alteration.
  • Variable behavior; can recur.

🔍 Did You Know?

The recognition that BRAF V600E mutations are present in ~70% of pleomorphic xanthoastrocytomas and many gangliogliomas, plus a subset of pilocytic astrocytomas, has opened a new chapter in pediatric neuro-oncology. The BRAF V600E mutation produces a constitutively active BRAF kinase that drives the MAPK pathway. BRAF inhibitors (dabrafenib, vemurafenib) — originally developed for melanoma — and MEK inhibitors (trametinib, selumetinib) directly block this oncogenic pathway. The combination of BRAF + MEK inhibition has produced dramatic responses in pediatric low-grade gliomas with BRAF V600E, sometimes shrinking tumors substantially with relatively manageable toxicity. The 2023 approval of dabrafenib + trametinib for pediatric BRAF V600E low-grade glioma marked a new era: a single mutation testing decision (BRAF V600E IHC) directly determines specific targeted therapy. For inoperable or progressive low-grade gliomas in children with BRAF V600E — once condemned to long-term observation or radiation with cognitive consequences — targeted therapy is now first-line. The lesson generalizes: pediatric brain tumors driven by single recurrent mutations in kinases (BRAF, FGFR, NTRK, ALK) are increasingly targetable with oral therapies, often with better tolerability than radiation/chemotherapy. The molecular workup for any pediatric brain tumor now must include BRAF V600E and the major fusion partners.

Pitfalls and Pearls

  • Pilocytic astrocytoma: CNS WHO grade 1; biphasic + Rosenthal fibers + EGBs; KIAA1549-BRAF fusion or BRAF V600E; cerebellar predilection.
  • Cystic + mural nodule + cerebellar in a child: classic pilocytic astrocytoma imaging.
  • Pilomyxoid astrocytoma: aggressive variant in infants; hypothalamic.
  • PXA: BRAF V600E ~70%; superficial temporal; pleomorphic + xanthomatous; reticulin-rich.
  • SEGA: tuberous sclerosis; foramen of Monro; mTOR inhibitors transformative.
  • Ganglioglioma: BRAF V600E; temporal lobe; epilepsy; surgery curative.
  • DNET: specific glioneuronal element; floating neurons; refractory epilepsy; resection curative.
  • Central neurocytoma: lateral ventricle young adult; calcified; synaptophysin+.
  • DIG/DIA: infants; large superficial desmoplastic.
  • Subependymoma: incidental; benign; adults.
  • Astroblastoma, MN1-altered: perivascular pseudorosettes; MN1 alteration.
  • BRAF V600E + MEK inhibitor combination (dabrafenib + trametinib): now approved for pediatric BRAF V600E low-grade glioma.
  • Everolimus / sirolimus: SEGAs in TSC.
  • Selumetinib (MEK inhibitor): NF1-associated optic pathway glioma.

References

  1. Louis DN, Perry A, Wesseling P, et al. The 2021 WHO Classification of Tumors of the Central Nervous System: a summary. Neuro Oncol. 2021;23(8):1231-1251.
  2. Bandopadhayay P, Bergthold G, London WB, et al. Long-term outcome of 4,040 children diagnosed with pediatric low-grade gliomas. Pediatr Blood Cancer. 2014;61(7):1173-1179.
  3. Hargrave DR, Bouffet E, Tabori U, et al. Efficacy and safety of dabrafenib in pediatric patients with BRAF V600 mutation-positive relapsed or refractory low-grade glioma. Clin Cancer Res. 2019;25(24):7303-7311.
  4. Jones DT, Hutter B, Jäger N, et al. Recurrent somatic alterations of FGFR1 and NTRK2 in pilocytic astrocytoma. Nat Genet. 2013;45(8):927-932.
  5. Krueger DA, Care MM, Holland K, et al. Everolimus for subependymal giant-cell astrocytomas in tuberous sclerosis. N Engl J Med. 2010;363(19):1801-1811.