Cutaneous localization of plasmablastic multiple myeloma with heterotopic expression of CD3 and CD4: Skin involvement revealing systemic disease.

Plasmablastic multiple myeloma is an uncommon morphological variant of multiple myeloma with aggressive clinical course and poor outcome. Its differential diagnosis includes plasmablastic lymphoma, a variant of diffuse large B-cell lymphoma with frequent extranodal presentation, which usually affects immunosuppressed patients and is virtually indistinguishable from plasmablastic multiple myeloma on the basis of histology solely. Differential diagnosis relies on close clinical-pathological correlation. Herein, the authors report a case of aggressive multiple myeloma occurring in a 48-year-old patient with pure plasmablastic morphology, expression of T-cell markers CD3 and CD4, and cutaneous involvement as first presenting sign. Heterotopic expression of T-cell markers has been described in literature for both plasmablastic multiple myeloma and plasmablastic lymphoma. The causative mechanisms underlying this aberrant phenotype have not yet been elucidated; nevertheless the possibility of this rare finding should be considered to avoid misinterpretations. Remarkably, despite occurring rarely, cutaneous involvement could be observed at an early stage or even be the first manifestation of disease in particularly aggressive forms of myeloma. As a consequence, the presence of cutaneous lesions should not favor a straightforward diagnosis of plasmablastic lymphoma. The importance of a correct differential diagnosis lies in its therapeutical implications.