Differentiation of plasma cell infiltrates in the bone marrow. A clinicopathological study on 80 patients including immunohistochemistry and morphometry.

In 80 patients immunohistochemical, morphometrical and clinical studies were performed on routinely referred trephine biopsies of the bone marrow showing an abnormal increase in plasma cells. From the approximately determined density of plasma cell infiltrates two main groups were distinguished, the first with an involvement exceeding 20% and the second with less than 10% of the total marrow area involved. The first group (n = 30; 324 +/- 130 plasma cells per square millimeter bone marrow) consisted of patients with frank malignant myeloma (MM) by clinical and histomorphological diagnosis. The second group (n = 50; 132 +/- 54 plasma cells per square millimeter bone marrow) with plasmacytic differentiation of infiltrates, had to be further divided into one component with evidence for initial or residual MM following chemotherapy (n = 27), another with obviously monoclonal gammopathy of undetermined significance--benign monoclonal gammopathy (BMG, n = 6), and a final set of cases with a reactive plasmacytosis mostly associated with an inflammatory condition (n = 17). There was an excellent agreement between the intracellular immunoglobulin staining as defined by the immunoperoxidase technique and the serum or urinary M-component detected by immunoelectrophoresis. In MM significant correlations were found between osteoclastic activity (number of osteoclasts specifically stained by acid phosphatase) per trabecular bone area, presence of lytic bone defects and the density of plasma cell infiltrates in the marrow. This latter feature corresponded well with the titer of secreted serum M-components measured by quantitative immunoelectrophoresis. Using morphological data alone, BMG cases could not be discriminated with any certainty from initial or residual plasmacytic MM. They consequently need a prolonged clinical follow up to clarify the nature of the lesions.