First Reported Case of Malignant Ectomesenchymoma with p.Leu122Arg Mutation in MYOD1 Gene: Extensive Intra- and Extracranial Tumor in a 15-Year-Old Female

Background Ectomesenchymomas (EMs) are extremely rare neoplasms composed of malignant mesenchymal components and neuroectodermal derivatives. They are described in a wide variety of locations, with the head and neck region being one of the most frequently involved areas. EMs are usually managed as high-risk rhabdomyosarcomas and have similar outcomes. Methods We present the case of a 15-year-old female with an EM that arose in the parapharyngeal space and extended into the intracranial space. Results Histologically, the tumor presented an embryonal rhabdomyosarcomatous mesenchymal component and the neuroectodermal component was constituted by isolated ganglion cells. Next-generation sequencing (NGS) revealed a p.Leu122Arg (c.365 T > G) mutation in the MYOD1 gene, a p.Ala34Gly mutation in the CDKN2A gene, and CDK4 gene amplification. The patient was treated with chemotherapy. She died 17 months after the debut of symptoms. Conclusion(s) To our knowledge, this is the first reported case in English literature of an EM with this MYOD1 mutation. We suggest combining PI3K/ATK pathway inhibitors in these cases. NGS should be performed in EMs cases to detect mutations with potential treatment options.