Microscopic Features: Testicular plasmacytomas are composed of diffuse sheets of immature plasma cells with an interstitial (intertubular) growth pattern. The periphery of the tumor often shows entrapped seminiferous tubules. The immunohistochemical profile is similar to that of multiple myeloma (strong expression of CD38, CD138, MUM1, and cytoplasmic light chains with kappa or lambda restriction; negative for CD19 and CD20). The differential diagnosis includes seminoma, spermatocytic tumor, and lymphomas with plasmacytic differentiation, including lymphoplasmacytic lymphoma, immunoblastic or plasmablastic large cell lymphoma, and marginal zone lymphoma of MALT type.
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