Cutaneous B-cell lymphoma with histologic features of mycosis fungoides.

Histologic examination of skin biopsy specimens from a 57-year-old man with a pruritic rash on his chest, abdomen, and thighs revealed a dense atypical dermal lymphoid infiltrate bordering the dermoepidermal junction and scattered intraepithelial lymphocytes. Histopathologic and clinical features were suggestive of mycosis fungoides. Immunophenotyping studies, however, identified CD20(+) B lymphocytes with aberrant expression of CD43. Clonal immunoglobulin gene rearrangement was demonstrated, and no clonal T-cell gene rearrangement was identified. Morphologic and immunophenotypic features were most consistent with extranodal marginal zone B-cell lymphoma. Systemic evaluation identified involvement of the bone marrow and possibly the peripheral blood, spleen, and splenic lymph nodes. Cutaneous manifestations were treated with narrowband ultraviolet B phototherapy for 3 months, resulting in complete resolution of the pruritus and rash. Primary and secondary cutaneous B-cell lymphomas seldom mimic mycosis fungoides histologically. Immunophenotyping studies were critical in correctly classifying this rare example of an epidermotropic B-cell lymphoma.