Cytopathologic Diagnosis of Extramedullary Hematopoiesis and Sclerosing Extramedullary Hematopoietic Tumor: A 10-Year Institutional Experience

Summary: Introduction: Extramedullary hematopoiesis (EMH) is defined as proliferation of hematopoietic elements outside of the bone marrow that is commonly but not always associated with hematologic diseases. While sclerosing extramedullary hematopoietic tumor (SEMHT) has overlapping clinical and morphologic features with EMH, SEMHT is usually less cellular and has more of solid mass appearance with dense fibrosis and atypical megakaryocytes compared to EMH. EMH and SEMHT are uncommon manifestations that are rarely reported in cytology. This study aims to investigate the reliability and utility of cytologic techniques in the diagnosis of EMH/SEMHT in different types of cytology samples at the author's institution. Materials and Methods: The laboratory information system was queried over a period of 10 years (2014-2024) to identify all cytology cases diagnosed on fluid cytology, FNA, and/or small-core biopsy with touch preparations as EMH and/or SEMHT. History of hematopoietic disorders, specimen location, specimen type, cytopathologic diagnosis, and ancillary studies were reviewed and correlated. Results: A total of 11 cases from 11 patients were identified. A history of transplant (3 patients), and history of hematopoietic disorders (1 MDS, 3 primary myelofibrosis, 1 CML and 1 essential thrombocytosis: 6 patients ) was noted. The specimen origins were distributed as follows: 1 peritoneal fluid, 2 pleural fluids, and 8 ST masses (4 paraspinal, 1 presacral, 1 peritoneal, 1 perigastric and 1 periportal lymph node). This corresponded to 3 fluid cytology samples, 5 FNAs, and 3 small core biopsies with touch preparations. 5 cases were diagnosed as atypical immature hematopoietic cells and megakaryocytes or trilineage hematopoiesis compatible with EMH, and 6 cases were diagnosed as negative for malignant cells with trilineage hematopoiesis and megakaryocytes compatible with EMH (5 cases) and SEMHT (1 case). Flow cytometry was performed on 5 cases, while immunohistochemical stains (CD61 and/or myeloid markers) were performed on 7 cases, with both ancillary studies confirming the diagnosis of EMH/SEMHT. Conclusions: Cytology can provide a reliable and accurate method for diagnosing EMH/SEMHT. EMH is less commonly encountered in serous effusions (27.3%) compared to their solid counterpart. Ancillary studies, including flow cytometry and immunohistochemistry, are reliable cytologic techniques that could aid in the diagnosis of EMH/SEMHT in various cytology samples.