Fulminant extraneural metastases in MYC-amplified Group 3 medulloblastoma: a pediatric case report.

Medulloblastoma (MB) with MYC amplification is associated with aggressive clinical behavior and a high risk of metastatic dissemination. Extraneural metastasis (ENM), although rare in the modern treatment era, remains a devastating manifestation linked to poor survival. We report a pediatric case of MB with high-level MYC amplification and NanoString-based molecular subgrouping confirming Group 3 identity in a 9-year-old girl who developed rapidly progressive and widespread ENMs involving the bone marrow, liver, lymph nodes, and peritoneum shortly after craniospinal irradiation. Planned adjuvant chemotherapy could not be initiated because of persistent cytopenias associated with marrow infiltration during disease progression. Targeted next-generation sequencing identified somatic alterations in PTEN, ARID2, and ERCC6, as well as a heterozygous germline SLX4 variant of uncertain clinical significance. While these findings do not establish a causal role in tumor progression, they further illustrate the molecular complexity of high-risk MB. The clinical course observed in this patient is consistent with prior reports linking MYC amplification to aggressive metastatic behavior. This case underscores the challenges of managing molecularly high-risk MB and highlights the importance of comprehensive molecular characterization and timely systemic therapy.