Myeloma with immunohistochemically different cell populations: implication for the validity of immunohistochemical assessment of lymphoma cell monoclonality.

A case of myeloma showed histologically a monotonous pattern of relatively mature myeloma cells and an IgG-kappa paraproteinemia. In contrast to ordinary stains, the immunoperoxidase reactions for kappa and gamma gave inhomogeneous results. Light and heavy chains were formed by clusters of different subpopulations of myeloma cells. Serial sections showed that kappa-positive clusters were gamma-negative and vice versa. Thus, this morphologically homogeneous myeloma turned out to be immunohistochemically heterogeneous. We assume that two subpopulations have developed that produced separately kappa- and gamma-chains. Therefore, these chains might have been circulating separately and not as complete IgG-kappa molecules. Guided by our findings, the results of other investigators are discussed. Some considerations on the validity of immunohistochemical demonstration of tumor cell monoclonality are given implicating the diagnostic equivocation of immunohistochemistry for non-Hodgkin's lymphomas, especially for so-called "immunocytomas".