Expanding the Phenotype of ALK-positive Histiocytosis: A Report of 2 Cases.

ALK-positive histiocytosis is a recently described rare histiocytic proliferative disorder of early infancy. When infants present with anemia, thrombocytopenia, and hepatosplenomegaly, this entity should be included in the differential diagnosis along with hemophagocytic lymphohistiocytosis, metabolic/storage diseases, hematopoietic malignancies, and autoimmune thrombocytopenia. We report 2 new cases of ALK-positive histiocytosis, one with kidney involvement and the other with extensive bone marrow involvement in addition to an overt liver disease. Renal involvement by ALK-positive histiocytosis has not been reported. The infiltrating histiocytes in this case showed Rosai-Dorfman disease-like morphology including emperipolesis. The histiocytes in the second case with extensive bone marrow involvement had foamy cytoplasm initially suggesting storage disease. Our 2 cases highlight previously unrecognized diversity of ALK-positive histiocytosis in clinical manifestation, organ involvement, and cytomorphologic features and further elucidate the diagnostic challenges of this rare entity.