Undifferentiated embryonal sarcoma of the liver with a novel MAD1L1::ERG fusion: a 20-year survival case report.

Undifferentiated embryonal sarcoma of the liver (UESL) is a rare, highly aggressive malignant mesenchymal tumor that predominantly affects children and typically carries a poor prognosis. We report a unique UESL case with long-term survival harboring a novel MAD1L1::ERG gene fusion. A 15-year-old female underwent segmentectomy for a 15-cm hepatic mass, without a precise diagnosis initially. From 2018 to 2025, she experienced multiple recurrences involving the liver, right kidney, and retroperitoneum. Histopathological examination across all recurrences consistently showed classic UESL features: pleomorphic spindle or polygonal cells in a myxoid stroma, frequent mitotic figures, bizarre multinucleated giant cells, and characteristic PAS-positive eosinophilic hyaline globules. Immunohistochemistry (IHC) was positive for vimentin. Targeted DNA next-generation sequencing (NGS) identified 23 genetic alterations, most notably a novel MAD1L1::ERG gene fusion (breakpoint: MAD1L1 Intron 18 and ERG Exon 13) alongside CCND2, FGFR3, MYC, SRC amplifications, and a TP53 mutation. Despite the tumor's highly aggressive behavior, the patient achieved long-term survival through repeated multi-visceral resections and systemic chemotherapy (VAC and IE). This report expands the molecular genetic profile of UESL by identifying a previously undescribed gene rearrangement.