Neuro / Head & Neck for Tuesday, June 2nd, 2026

Contributed by Emory University
Ayushi Gupta, Dr Darragh Brady, and Dr Kartik Reddy.

History

A 21-month-old female with Dandy–Walker phenotype and global developmental delay presented with decreased oral intake, intermittent fevers, and reduced activity. Prior neonatal MRI showed congenital brain abnormalities without mass lesions.

Images (Click any image to enlarge)

Day 1
21 months; Stable Dandy–Walker malformation. New heterogeneously hypodense lesion in the left cerebellar hemisphere concerning for mass. Additional hyperattenuating nodular areas with curvilinear calcification in the right posterior fossa, indeterminate (lesion vs vascular/dysplastic). No significant mass effect or hydrocephalus.
21 months; Right extra-axial diffusion-restricting cerebellar mass (epidermoid vs high-grade neoplasm). Left intra-axial cystic–solid cerebellar mass with low-grade features. No metastases. Background complex brain malformations (Dandy–Walker spectrum, polymicrogyria, septal agenesis)

Question

What is the most likely explanation for two cerebellar masses with differing imaging characteristics in this patient?

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Correct answer

Synchronous occurrence of two distinct primary cerebellar tumors (low-grade glioma- Pilocytic astrocytoma and high-grade embryonal tumor- Medulloblastoma) arising in a dysplastic cerebellum.

Discussion

Imaging demonstrated two distinct cerebellar lesions: a right-sided mass with diffusion restriction and avid enhancement, suggestive of a high-grade tumor, and a left-sided cystic and solid lesion without diffusion restriction, suggestive of a low-grade tumor. There was no evidence of metastatic disease.

Histopathologic and molecular evaluation confirmed two biologically independent primary tumors. The left cerebellar lesion was a pilocytic astrocytoma (CNS WHO grade 1) with a KIAA1549::BRAF fusion, consistent with a MAPK-driven low-grade glioma. The right cerebellar lesion was a desmoplastic/nodular medulloblastoma (CNS WHO grade 4), SHH-activated subtype.

The synchronous presence of two distinct tumor types within the cerebellum suggests an underlying developmental susceptibility. The cerebellum undergoes tightly regulated growth during fetal and early postnatal life, governed by signaling pathways such as SHH and MAPK that control neuronal and glial progenitor proliferation and differentiation. Disruptions in these pathways may not only result in structural malformations but also alter progenitor cell behavior, increasing susceptibility to oncogenic transformation.

SHH-activated medulloblastomas arise from granule cell precursors, whereas pilocytic astrocytomas originate from gliogenic progenitors, reflecting distinct developmental lineages within the cerebellum. The presence of dysplastic neuronal elements adjacent to the low-grade tumor supports the concept that tumorigenesis occurred within an intrinsically abnormal microenvironment.

The coexistence of Dandy–Walker phenotype with additional anomalies, including polymicrogyria and tectal dysplasia, suggests a broader disturbance of early brain development. These findings support the concept that congenital malformations and pediatric brain tumors may represent related manifestations of disrupted developmental signaling, with early perturbations predisposing to multiple, independent neoplastic processes within the same dysplastic cerebellum.

Additional images

References

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