The disease is characterized by a lymphoid infiltrate in the splenic white pulp that grows in a nodular patter replacing preexisting follicles.482 A variable degree of red pulp infiltration is also often present. Small lymphocytes are predominant in central areas, while medium-size cells resembling splenic marginal zone lymphocytes are present in the periphery. Plasmacytic differentiation as well as, rarely, clusters of plasma cells can be present.
The neoplastic cells show typical positivity for surface (and sometimes cytoplasmic) immunoglobulins and pan-B antigens (CD19, CD20, and CD22); they usually coexpress CD11c, and lack CD5, CD10, and CD23 expression.
Up to two-thirds of the patients with splenic MZL present the characteristic circulating villous lymphocytes, with short cytoplasmic projections. When these are more than 20% of the lymphocyte count, the term "splenic lymphoma with villous lymphocytes" is commonly used.83
Bone marrow is usually involved, even in cases with no circulating neoplastic cells. The pattern of infiltration is typically nonparatrabecular, intrasinusoidal only. However, this pattern of bone marrow involvement can be found also in other small-cell lym-phomas.84-86
When biopsied, the liver is usually involved with a nodular infiltration of portal tracts, and hilar splenic lymph node involvement is common. According to the WHO classification, the peripheral lymph node involvement is typically absent.87 Transformation to a diffuse large B-cell lymphoma occurs in about 15% of the cases.88
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