Myeloid sarcoma presenting as an isolated pancreatic mass in a 3-year-old child

Myeloid sarcoma (MS) is an extramedullary tumor of myeloid precursor cells, frequently associated with acute myeloid leukemia (AML), and rarely occurring in isolation. We present a child with obstructive jaundice secondary to a pancreatic head mass. Initial imaging was consistent with pancreatic hematoma; however, continued symptoms led to endoscopic ultrasound (EUS)-guided biopsies that raised concern for autoimmune pancreatitis. Further histological and cytogenetic analysis confirmed pancreatic MS associated with a RUNX1::RUNX1T1 fusion. Bone marrow evaluation was negative by conventional diagnostic methods; however, reverse transcriptase with qualitative real-time polymerase chain reaction (RT-qPCR) detected RUNX1::RUNX1T1 transcripts. Chemotherapy achieved both radiologic resolution and disappearance of RUNX1::RUNX1T1 transcripts within the bone marrow. This represents one of the first reported cases of isolated pancreatic MS in a child, without progression to AML. Our case highlights the diagnostic challenges of pancreatic masses, the underutilization of EUS-guided biopsy in pediatrics, and the use of RT-qPCR for both diagnosis and disease resolution.