Medulloblastoma in concurrence with metastatic gastrointestinal stromal tumor: what's the GIST? Illustrative case.

backgroundMedulloblastomas (MBs) are the most common malignant brain tumor in pediatrics and are associated with several germline risk disorders such as Li Fraumeni, Gorlin, and Turcot syndromes. While most arise idiopathically, inherited cancer syndromes contribute to a minority of cases and have implications in both treatment and surveillance. Despite being the most common mesenchymal tumor of the gastrointestinal tract, gastrointestinal stromal tumors (GISTs) are exceedingly rare in children and do not have a known association with cancer syndromes that predispose one to MB. The authors present a unique pediatric case of concurrent MB and metastatic GIST and raise the question of a possible shared genetic etiology, as there stands no prior report of the two occurring simultaneously within the literature. OBSERVATIONS: A 17-year-old female with a history of metastatic GIST was found to have a growing cerebellar lesion on MRI following an emergency department visit for persistent nausea, imbalance, and intermittent headaches. Suboccipital resection revealed a distinct second primary tumor, which was identified as SHH-activated TP53 wildtype MB. This case highlights an unusual concurrence of two rare malignancies in a pediatric patient. LESSONS: This unique presentation of an SHH-activated MB in a pediatric patient with metastatic GIST demonstrates the potential for shared genetic susceptibilities between embryonal and mesenchymal tumors. The authors highlight the importance of germline testing in childhood cases of multiple primary malignancies, especially with SHH-activated MBs to identify possible underlying hereditary cancer syndromes. https://thejns.org/doi/10.3171/CASE25749.