Non-Hepatosplenic Extramedullary Hematopoiesis Masquerading as Metastatic Malignant Melanoma.

Non-hepatosplenic extramedullary hematopoiesis (NHS-EMH), the formation of blood cellular elements outside the medulla of the bone marrow and outside the liver and spleen, has been noted among patients with myeloproliferative neoplasms and other serious hematological diseases. However, NHS-EMH is rarely identified among individuals without concurrent hematological disease. Since the radiologic features of NHS-EMH are nonspecific, lesions may be mistaken for metastatic disease when observed in patients with known solid tumors. We report an unusual case of a patient with a simultaneous presentation of malignant melanoma and multiple NHS-EMH lesions. The biopsy revealed trilineage hematopoiesis resembling normal bone marrow tissue, in the absence of abnormalities of peripheral blood counts or presence of driver mutations associated with myeloproliferative neoplasms. The biopsy results were critical in downstaging the patient and thus permitted avoidance of unnecessary malignant melanoma therapy. This case emphasizes the importance of surgical biopsy of suspect lesions when treatment strategies will be impacted.