Histiomonocytic malignancy. A spectrum of disease in an 11-month-old infant

The initial pathologic diagnosis in an 11-month-old girl presenting with a suprarenal mass was true histiocytic lymphoma. The histiocytic nature of the cells was verified by ultrastructural, histochemical, and immunologic studies. The subsequent course featured widespread dissemination as both tumorous masses and diffuse tissue infiltrates, including extensive soft tissue, leptomeningeal, and bone marrow involvement, with a terminal histiomonocytic leukemic phase. Subsequently, this tumor was reclassified as malignant histiocytosis with atypical features, and this case exemplifies the difficulties in classifying some malignant histiomonocytic neoplasms. The overlapping clinical, pathologic, and theoretic features of true histiocytic lymphoma, malignant histiocytosis, and histiomonocytic leukemia are discussed in the context of this case.