Extramedullary manifestation of multiple myeloma (systemic plasmacytoma) that simulates hemangioma

We describe 2 cases of a unique type of extramedullary manifestation of multiple myeloma (systemic plasmacytoma) that presented as subcutaneous nodules and mediastinal mass, respectively. Both lesions had a similar morphologic appearance, with dilated vascular-like lumina that was separated by thin fibrovascular septa, filled with erythrocytes, and lined by mature and immature plasma cells and plasmacytoid cells. The plasma and plasmacytoid lining cells showed kappa light chain restriction in both cases, consistent with a B-cell monoclonal process. The lining cells were also focally positive for epithelial membrane antigen but were negative for endothelial cell markers. Abundant delicate capillaries were seen in the septa that separated the vascular lumina, mimicking a vascular tumor. Furthermore, we believe that our cases are different from the previously described blood lakes in a plasmacytoma by the presence of well-formed fibrovascular septa that separated the vascular-like spaces. Neoangiogenesis propagated by myeloma cells may contribute to this unusual morphologic manifestation of extramedullary manifestation of multiple myeloma.