Adult Erythroblastic Sarcoma With PCM1::JAK2 Fusion and a Novel NOP10::NUTM1 Fusion Complicated by Secondary HLH.

Erythroblastic sarcoma is a rare and aggressive hematologic malignancy presenting as a mass-forming extramedullary proliferation of immature erythroid cells. Myeloid/lymphoid neoplasms with JAK2 rearrangement, most often PCM1::JAK2, may show eosinophilia, myelofibrosis, and expansion of immature erythroid precursors. NUTM1 rearrangements, initially recognized as defining alterations of NUT carcinoma, have subsequently been identified in selected hematologic malignancies but remain exceptionally rare in myeloid neoplasms. Here, we report an adult erythroblastic sarcoma harbouring concurrent PCM1::JAK2 and a novel NOP10::NUTM1 fusion, accompanied by partial chromosome 8p deletion and complicated by secondary hemophagocytic lymphohistiocytosis. To our knowledge, this is the first reported adult erythroblastic sarcoma with concurrent PCM1::JAK2 and NOP10::NUTM1 fusions. This case extends the molecular spectrum of erythroblastic sarcoma and underscores the value of integrated morphologic, immunophenotypic, cytogenetic, and genomic assessment in diagnostically challenging erythroid neoplasms.