IgG4 expression in cutaneous marginal zone lymphoma with plasmacytic differentiation and localized amyloid deposition: A useful clue to cutaneous origin

Primary cutaneous lymphomas include T-cell and B-cell neoplasms limited to skin without evidence of extracutaneous disease. Cutaneous follicle center cell lymphoma is the most common cutaneous B cell lymphoma, comprising 12% of cases. Primary cutaneous marginal zone B-cell lymphoma (PCMZL) accounts for 7% of all skin lymphomas and comprises 25% to 30% of cutaneous B cell lymphoma. Cutaneous diffuse large B-cell lymphoma of the leg type is the least common at about 4% of cases.1 PCMZL typically presents as pink-purple, solitary or grouped papules, plaques, and nodules on the trunk and upper extremities of adults in the fifth to sixth decade with a male predominance. Biopsy typically finds a vaguely nodular nonepidermotropic, deep dermal (bottom-heavy), subcutaneous and/or periadnexal polymorphous lymphoid infiltrate. Small lymphocytes surround germinal centers with either centrocytelike or plasmacytoid morphology. Plasma cells are often increased. Lymphoma cells express CD20 and CD79a and are negative for CD5, CD10, and Bcl-6. If plasma cells are increased, they exhibit immunoglobulin light chain restriction. Clonality can be identified in about 80% of cases.2 PCMZL and mucosa-associated lymphoid tissue (MALT) lymphomas may be preceded by chronic antigen stimulation and are the most indolent tumors in this group. Only 4% of patients have extracutaneous disease, and less than 1% of patients die of disease; therefore, distinguishing PCMZL from marginal zone lymphoma (MZL) with secondary skin involvement is clinically important.1, 2