A 67-year-old man with a history of advanced fibrotic stage primary myelofibrosis on ruxolitinib (MPL mutated [exon 10; c.1544G>T; p.Trp515Leu per next-generation sequencing]), underwent prostatectomy for prostate adenocarcinoma and left inguinal lymphadenectomy of a 1.7 × 1.0 × 0.5-cm lymph node for increased activity on imaging. On histologic examination, there was a densely fibrotic 1.2 × 0.8-cm lesion with distention of lymph node sinuses by numerous clusters of large multinucleated cells resembling atypical megakaryocytes (panels A-B, hematoxylin and eosin stain; 40× objective), accompanied by scattered erythroid and granulocytic precursors. Immature blasts were not appreciated. By immunohistochemistry, CD61 highlighted the atypical megakaryocytes (panel C, CD61 stain; 40× objective). Myeloperoxidase (MPO) and hemoglobin A (HgA) highlighted scattered granulocytic and erythroid precursors, respectively (panels D-E, MPO and HgA stain, respectively; 40× objective). There were occasional KIT-positive cells that lacked CD34 expression (panel F, CD117 stain; 40× objective). Immunohistochemical stains for epithelial and sarcomatous tumors were negative. These findings are characteristic of sclerosing extramedullary hematopoietic tumor (SEMHT).

SEMHT is a rare neoplasm characterized by mature hematopoietic elements that form fibrotic extramedullary tumors in patients with advanced-stage myeloproliferative neoplasms. Characteristic features, including prominent atypical megakaryocytes and dense collagen fibrosis, distinguish it from extramedullary hematopoiesis. SEMHT may be mistaken for sarcomas, particularly sclerosing liposarcoma, and Hodgkin lymphoma when the clinical history is unknown. This case emphasizes the importance of reviewing previous hematological history and clinicopathological correlation.