Embryonal Tumors (Medulloblastoma & Others)

Embryonal tumors of the CNS are aggressive malignancies that arise from undifferentiated precursor cells, predominantly in children. The most common is medulloblastoma. The 2021 WHO classification has refined embryonal tumor categories around molecular subgroups, particularly in medulloblastoma where the four molecular subgroups (WNT, SHH, Group 3, Group 4) have transformed prognostication and treatment selection. This page covers medulloblastoma, atypical teratoid/rhabdoid tumor (AT/RT), embryonal tumor with multilayered rosettes (ETMR), and pineoblastoma.

Medulloblastoma

The most common malignant brain tumor of childhood; cerebellar location.

2021 WHO Molecular Subgroups

WNT-activated medulloblastoma

  • ~10% of medulloblastomas.
  • Older children, adolescents.
  • CTNNB1 (β-catenin) mutations or APC mutations.
  • Nuclear β-catenin IHC positive.
  • YAP1, GAB1 IHC positive.
  • Best prognosis: > 90% 5-year survival.
  • Treatment de-escalation increasingly used.

SHH-activated medulloblastoma

  • ~30%.
  • Bimodal age: infants/young children + adults.
  • Activated sonic hedgehog signaling: PTCH1, SMO, SUFU mutations.
  • Often desmoplastic/nodular or extensive nodularity histology.
  • GAB1 positive; YAP1 positive.
  • Prognosis variable: TP53-mutant SHH worse; TP53-wildtype SHH better.
  • SMO inhibitors (vismodegib, sonidegib) in trials.

Group 3 medulloblastoma

  • ~25%.
  • Infants, young children.
  • MYC amplification often.
  • Photoreceptor / GABAergic gene expression.
  • Large cell / anaplastic histology common.
  • Worst prognosis: < 60% 5-year survival.
  • High metastasis rate.

Group 4 medulloblastoma

  • ~35% (most common).
  • Older children, adolescents; males predominate.
  • MYCN amplification subset.
  • KDM6A mutations.
  • Intermediate prognosis.

Histologic Variants

  • Classical (lobular/nodular pattern, Homer Wright rosettes, dense small blue cells).
  • Desmoplastic/nodular (reticulin-rich; SHH-activated typically).
  • Extensive nodularity (SHH-activated, infant).
  • Large cell / anaplastic (worse prognosis; Group 3 often).

Imaging

  • Posterior fossa midline mass arising from vermis.
  • Restricted diffusion (highly cellular).
  • Often heterogeneous enhancement.
  • Hydrocephalus common.
  • Leptomeningeal spread possible.

Clinical

  • Ataxia, vomiting, headache, raised ICP signs.
  • Cranial nerve palsies if extension.
  • Need staging with spine MRI + CSF cytology (leptomeningeal seeding).

Treatment

  • Surgery: maximal safe resection.
  • Craniospinal radiation (over age 3-5 typically; younger require modification due to neurocognitive risk).
  • Chemotherapy (vincristine, cisplatin, cyclophosphamide, lomustine, etoposide).
  • Molecular subgroup-stratified treatment increasingly used.
  • 5-year survival: ~75% overall; varies by subgroup and risk stratification.

Atypical Teratoid/Rhabdoid Tumor (AT/RT)

Clinical

  • Highly aggressive embryonal tumor.
  • Most common in children under 3 years.
  • Posterior fossa or supratentorial.
  • Rapid progression, often poor prognosis.

Pathology

  • SMARCB1 (INI1) loss: defining genetic feature; loss of nuclear INI1 IHC.
  • SMARCA4 (BRG1) loss: rare alternative.
  • Rhabdoid cells: large cells with eosinophilic cytoplasm, paranuclear inclusion-like structures, eccentric nuclei.
  • Mixed embryonal + epithelial + mesenchymal + neural elements.
  • Loss of INI1 IHC is diagnostic.

Treatment

  • Surgery + intensive multimodal therapy.
  • 5-year survival < 50% historically; intensive regimens improving.
  • Targeted therapies (EZH2 inhibitor tazemetostat for INI1-deficient tumors) in trials.

Embryonal Tumor with Multilayered Rosettes (ETMR), C19MC-Altered

Pathology

  • C19MC microRNA cluster amplification.
  • Multilayered rosettes (ependymoblastomatous).
  • Often called “embryonal tumor with abundant neuropil and true rosettes” historically.
  • Aggressive; young children.

Pineoblastoma

  • Embryonal tumor of pineal region.
  • Children, sometimes adults.
  • Highly aggressive.
  • Associated with DICER1 syndrome and retinoblastoma (RB1) germline mutations.
  • Treatment: surgery + craniospinal radiation + chemotherapy.

Other Embryonal Tumors

  • CNS embryonal tumor, NOS: heterogeneous group when not classified into specific entities.
  • Choroid plexus carcinoma: aggressive choroid plexus tumor; Li-Fraumeni associated.
  • Atypical choroid plexus papilloma: intermediate behavior.

🔍 Did You Know?

The transformation of medulloblastoma from a single histologic disease into four molecularly distinct subgroups is one of the great success stories of integrative cancer genomics. The four subgroups — WNT, SHH, Group 3, Group 4 — have such different biology, prognosis, and emerging treatment paradigms that 21st-century pediatric oncology essentially treats them as different diseases. WNT-activated medulloblastomas have > 90% cure rate and are now candidates for treatment de-escalation (less radiation, less chemotherapy) to preserve cognitive function. SHH-activated tumors have variable prognosis depending on TP53 status and may respond to hedgehog pathway inhibitors. Group 3 tumors have the worst prognosis and intense treatment intensification is being studied. Group 4 has intermediate behavior. The transition to molecular classification was driven by recognition that prognosis based on histologic features (large cell/anaplastic vs classic vs desmoplastic) underperformed the prognosis based on molecular subgroup. The 2021 WHO classification embeds the molecular subgroups at the core of medulloblastoma diagnosis, and the integrated diagnosis combines histologic and molecular features. The therapeutic implication: the same disease “medulloblastoma” treated identically with the standard surgery + radiation + chemotherapy in the 1990s now receives subgroup-tailored regimens that vary by an order of magnitude in intensity and cumulative toxicity. The lesson: brain tumor pathology in 2026 is fundamentally about molecular classification driving precision medicine in pediatrics as it has in adult gliomas.

Pitfalls and Pearls

  • Medulloblastoma: most common pediatric malignant brain tumor; cerebellar.
  • WNT-activated medulloblastoma: best prognosis (> 90% 5-year survival); β-catenin nuclear; older children.
  • SHH-activated medulloblastoma: variable; TP53 status critical (TP53-mutant worse).
  • Group 3 medulloblastoma: worst prognosis; MYC amplification.
  • Group 4 medulloblastoma: intermediate; male predominance.
  • Large cell/anaplastic histology: worse prognosis (Group 3).
  • Desmoplastic/nodular: SHH typically; can be better in infants with extensive nodularity.
  • AT/RT: SMARCB1 (INI1) loss; rhabdoid cells; young children; aggressive.
  • INI1 loss on IHC: defines AT/RT.
  • ETMR: C19MC amplification; multilayered rosettes.
  • Pineoblastoma: aggressive; DICER1 and RB1 germline mutations.
  • Craniospinal radiation: standard for medulloblastoma; cognitive consequences; proton therapy increasingly used.
  • SMO inhibitors (vismodegib, sonidegib): SHH medulloblastoma trials.
  • EZH2 inhibitor (tazemetostat): AT/RT trials.
  • Choroid plexus carcinoma: TP53 (Li-Fraumeni) association.

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. Northcott PA, Korshunov A, Witt H, et al. Medulloblastoma comprises four distinct molecular variants. J Clin Oncol. 2011;29(11):1408-1414.
  3. Cavalli FMG, Remke M, Rampasek L, et al. Intertumoral heterogeneity within medulloblastoma subgroups. Cancer Cell. 2017;31(6):737-754.e6.
  4. Ho B, Johann PD, Grabovska Y, et al. Molecular subgrouping of atypical teratoid/rhabdoid tumors-a reinvestigation and current consensus. Neuro Oncol. 2020;22(5):613-624.
  5. Fruhwald MC, Biegel JA, Bourdeaut F, Roberts CW, Chi SN. Atypical teratoid/rhabdoid tumors-current concepts, advances in biology, and potential future therapies. Neuro Oncol. 2016;18(6):764-778.
  6. Robinson GW, Rudneva VA, Buchhalter I, et al. Risk-adapted therapy for young children with medulloblastoma (SJYC07). Lancet Oncol. 2018;19(6):768-784.