Alveolar airspace and pulmonary artery involvement by extramedullary hematopoiesis: a unique manifestation of myelofibrosis.

Pulmonary extramedullary hematopoiesis is a rare manifestation of myelofibrosis. We encountered a unique case of pulmonary extramedullary hematopoiesis occurring in a 59-year-old white man, where in addition to the typical foci of interstitial hematopoietic cells, a surgical lung biopsy showed airspace and arterial wall involvement. Airspace foci were associated with acute and organizing alveolar hemorrhage, while within arteries the hematopoietic elements had a striking predilection for the vascular intima. The hematopoietic foci included erythroid precursors, myeloid precursors, and megakaryocytes, which were immunoreactive with hemoglobin, myeloperoxidase, and CD61, respectively. Whether extramedullary hematopoiesis represents in situ embryonic stem cell differentiation or a compensatory seeding of hematopoietic cells from the bone marrow remains to be elucidated. However, familiarity with these findings in the lung could be helpful in uncovering occult hematological disorders accompanied by extramedullary hematopoiesis. Extramedullary hematopoiesis should also be considered as a cause of pulmonary hemorrhage, especially in the setting of myelofibrosis.