Serum Free Light Chain Only Myeloma with Cytoplasmic IgM

Abstract

In the past decade, the serum free light chain (FLC) immunoassays have become widely available enabling greater sensitivity in the diagnosis and management of monoclonal light chain diseases. Here, we describe a rare case of serum free light chain only myeloma with cytoplasmic IgM. A 75-year-old woman presented with a progressively worsening lumbosacral pain. FDG PET/CT images showed increased FDG uptake in the sacral mass, vertebral bodies, and ribs. Laboratory data found hypogammaglobulinemia and the bone marrow aspirate revealed only 2.2% of plasma cells. The serum and urine protein electrophoresis did not detect a monoclonal band. However, the serum FLC immunoassays reported an abnormal kappa/lambda ratio (0.001) indicating the presence of monoclonal lambda FLC. The sacral tumor biopsy revealed proliferation of plasma cells and immunohistochemical staining showed that the plasma cells were positive for CD138, IgM, and lambda light chain but negative for CD20. This case may have previously been described as a nonsecretory IgM myeloma but recently would be identified as free light chain only myeloma. The immunohistochemical and genetic features of the clonal plasma cells in free light chain only myeloma need to be further investigated to better understand the relevance and incidence of this myeloma type.

Figure 1

Clinical course of the patient and response to treatment. Difference between involved and uninvolved free light chain (dFLC) and κ/λ ratio are reported in the graph alongside the clinical course and treatment regimens.

Figure 2

Coronal and axial fused ¹⁸F-FDG PET/CT images. (a) Initial diagnosis of myeloma, multiple foci of metabolic activity. (b) Extension of lesions and new lesions following relapse. (c) Complete metabolic resolution of myeloma. (d) In initial diagnosis of myeloma, PET/CT image shows ¹⁸F-FDG avid lytic lesions sacrum and iliac bone involvement. (e) In relapsed myeloma, PET/CT image shows developed ¹⁸F-FDG avid iliac and sacral bone lesions. (f) After 10 months undergoing LEN/DEX therapy, PET/CT image shows complete resolution of the metabolic activity.

Figure 3

Bone marrow and specimen from the sacral mass. ((a), (b)) Immunostaining for CD138 in the bone marrow clot section showing microcluster infiltration of plasma cells (x40, x200). (c) Bone marrow aspiration smear showing atypical plasma cells (May-Giemsa stain x600). ((d)–(j)) Specimen of sacral mass. (d) Hematoxylin and eosin (H&E) stain showing diffuse infiltration of plasma cells. Immunohistochemical stain showing (f) IgM+, (h) lambda light chain+, (i) CD138+, (e) IgG−, (g) kappa light chain−, and (j) CD20−.