A clinicopathologically distinctive primary splenic histiocytic neoplasm. Demonstration of its histiocyte derivation by immunophenotypic and molecular genetic analysis.

We describe a primary splenic neoplasm composed of cytomorphologically malignant-appearing erythrophagocytic histiocytoid cells reminiscent of those seen in malignant histiocytosis. However, this neoplasm displayed certain distinctive clinicopathologic features--including localization to the spleen, where it grew as separate discrete nodules--that distinguish it from all previously reported cases of malignant histiocytosis. The cells expressed a monocyte/histiocyte immunophenotype and lacked clonal immunoglobulin and T-cell receptor beta-chain gene rearrangements. Our results suggest that this neoplasm represents a clinicopathologically distinctive and possibly unique tumor derived from the tissue macrophage lineage.