Lymph node involvement by splenic marginal zone lymphoma: morphological and immunohistochemical features

Splenic marginal zone lymphoma (SMZL) has recently been proposed as a distinctive type of low-grade B-cell lymphoma. Although there is general agreement that this entity exists, its precise definition is blurred by uncertainty in differential diagnosis from other low-grade B-cell lymphomas. There is even more uncertainty as to the histology of splenic hilar and peripheral lymph nodes involved by SMZL. We therefore reviewed the histological and immunohistochemical features of 19 of these lymph nodes (14 hilar and five peripheral) from 14 cases of classical SMZL and compared them with the features of lymph nodes involved by other B-cell lymphomas. The morphology and immunohistology of the lymph nodes resemble those found in the white pulp of the spleen, showing a distinctive pattern, different from that which is observed in other B-cell lymphomas. In these cases, the overall architecture of the lymph nodes is effaced and replaced by a nodular infiltrate, although the sinuses are preserved in most hilar lymph nodes. Some of the nodules contain a central reactive follicular center, around which there is a broad zone of small lymphocytes. In other cases, the central area is partially infiltrated or, more commonly, totally replaced by these small lymphocytes, which in the periphery of the nodules showed a pale, slightly larger cytoplasm. Scattered nucleolated blasts are present, largely confined to the periphery of the nodules. The tumoral cells express immunoglobulin (Ig)D, IgM, and Ig light chain restriction and show a low proliferation fraction. These findings confirm that SMZL is a real entity, and not merely a morphological pattern of splenic infiltration by different types of low-grade B-cell lymphoma.