#00060966

A 65-year-old woman presented with a 21-month history of persistent anemia and progressive leukocytosis and thrombocytosis without intervention (hemoglobin, 10 g/dL; white blood cell count, 28.3 × 10 9/L; platelets, 702 × 10 9/L). Bone marrow aspirate revealed 2% blasts and dyserythropoietic megaloblastoid change and 60% ring sideroblasts (panel A, original magnification ×1000; iron stain). Megakaryocytic clustering and panhyperplasia were seen on the hypercellular (90%) biopsy (panel B, original magnification ×200; hematoxylin and eosin stain). Karyotyping showed 46,XX[20]. Bone marrow next-generation sequencing using a targeted 35-gene panel revealed JAK2 V617F (45%; panel C), SF3B1 K700E (46%; panel D), and TET2 L1081* (100%).This case exemplifies a myelodysplastic syndrome/myeloproliferative neoplasm with ring sideroblasts and thrombocytosis (MDS/MPN-RS-T), a full entity under the 2016 World Health Organization (WHO) classification, previously provisionally known as refractory anemia with ring sideroblasts associated with marked thrombocytosis. The 2016 WHO diagnostic criteria include anemia with dyserythropoiesis, ≥15% ring sideroblasts, persistent thrombocytosis (≥450 × 10 9/L), and a spliceosome gene SF3B1 mutation or, if absent, lack of confounding cytotoxic and/or growth factor therapy. SF3B1 mutation is seen in 70% to 90% of MDS cases with ring sideroblasts and MDS/MPN-RS-T. In MDS/MPN-RS-T, it commonly coexists with a JAK2 V617F (50% to 65%), or less commonly a CALR or MPL mutation (<10%). The concurrence of SF3B1 with an MPN driver mutation strongly supports a diagnosis of MDS/MPN-RS-T and likely accounts for its mixed MDS/MPN phenotype.