A 59-year-old woman with a known history of JAK-2⁺ (V617F) polycythemia vera (PV) previously managed with venesections was referred for possible progression to post-PV myelofibrosis on the basis of a leucoerythroblastic blood film, raised lactate dehydrogenase (LDH), and newly palpable splenomegaly (palpable at 1 cm below the rib cage). The patient had a significant history of metochondromatosis (a rare autosomal disorder resulting in enchondromas and osteochondroma-like lesions). Bone marrow trephine demonstrated infiltration, with numerous chondromas (panel A; hematoxylin and eosin [H&E] stain; original magnification of bone marrow trephine ×20; black arrow indicates chondromas). Only small areas of hematopoiesis remained and had preserved trilineage hematopoiesis (panel B; H&E stain; original magnification ×40), and reticulin stain demonstrated no increased reticulin deposition or fibrosis (panel C; reticulin stain; magnification ×40). She therefore did not meet the diagnostic criteria for post-PV myelofibrosis, with splenomegaly likely a result of extramedullary hematopoiesis.

Post-PV myelofibrosis is diagnosed on the basis of both clinical features (eg, anemia or loss of requirement for venesection, splenomegaly, or constitutional symptoms) and pathological/histological findings (eg, grade 2-3 marrow fibrosis or raised LDH). Clinicians should be aware that other conditions can cause a similar picture, including marrow infiltration with both benign (as in our case) and malignant tumors.