Differential Diagnosis

The most relevant differential diagnoses are primary cutaneous follicle center lymphoma (FCL), immunocytoma and cutaneous B-cell pseudolymphoma (B-

Primary cutaneous marginal zone B-cell lymphoma

Clinical features Adults aged over 40 years

Red to violaceous infiltrated plaques or nodules with an erythematous periphery on trunk and arms Histological features

Nodular or diffuse infiltrate of small- to medium-sized lymphocytes with pale cytoplasm Immunophenotype

CD19+, CD20+, CD22+, CD79a+, CD5-, CD10-, CD23-, bcl-6-, bcl-2+. Regular and irregular networks of CD21+ follicular dendritic cells Molecular biology

IgH genes are monoclonally rearranged in the majority (>70%). t(14;18) translocation usually not present Laboratory evaluation

Search for Borrelia infection Treatment

Antibiotics, excision, ionizing radiation, rituximab Clinical course and prognosis Excellent prognosis, tendency to recur

PSL). All three diseases also have a good outlook. By definition, B-cell PSL does not spread to extracutaneous sites and never runs a fatal course. In contrast to MZL, only reactive germinal centers with distinct mantle zones, regular networks of CD21+ FDC and tingible body macrophages are found. Clonality of IgH genes or restricted expression of Ig light chains is not found in the vast majority in B-PSL. Clonal cases of B-PSL may even represent MZL or FCL. Marginal zone lymphoma and FCL share many clinical as well as histologic features. The inverse pattern with darker centers of the nodular infiltrates of MZL, sheets or collections of plasma cells, and the presence of numerous cells with lymphoplasmacytoid or mono-cytoid morphology argue for MZL. Immunohistochemistry may be helpful for differentiating between these two forms. FCL expresses bcl-6 uniformly and usually expresses CD10 and bcl-6, while MZL is negative for bcl-6 and CD10, but bcl-2 positive (13). Colonized follicles in MZL contain numerous bcl-6- and bcl-2+ cells. Coexpression of CD20 and CD43 is seen in many cases of FCL, but not found in MZL (3). The issue of the nosologic relationship of MZL and immunocytoma (lymphoplasmacytic lymphoma) has not yet been clarified (14). In immunocytoma diffuse infiltrates of lymphoplasmacytoid cells and Dutcher bodies (intranuclear PAS-positive inclusions) are typically found. However, Dutcher bodies are not completely specific for immunocytoma. A further differential diagnostic consideration is cutaneous follicular hyperplasia with monotypic plasma cells which may be closely related or even identical to MZL.

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