Smoldering Waldenström’s Macroglobulinemia (SWM) is defined as a serum IgM monoclonal protein ≥3 g/dL and/or ≥10% bone marrow lymphoplasmacytic infiltration but no evidence of constitutional symptoms, symptomatic anemia, or hyperviscosity. Forty-eight patients with SWM were identified at Mayo Clinic from 1974 to 1995. Patients with a diagnosis of chronic lymphocytic leukemia or lymphoma or history of any antineoplastic therapy before diagnosis of SWM were excluded. The median age at diagnosis was 63 years (range 39–87 years). Only one patient (2%) was < 40 years of age, but 14 percent were < 50 years old. Thirty-two (67%) were men, and 16 (33%) were women. At diagnosis, hepatomegaly was noted in 10%, splenomegaly in 4%, and lymphadenopathy in 8%. The initial hemoglobin level ranged from 8.7 to 15.3 g/dL (median 11.8). The anemia in all 4 patients with an initial hemoglobin < 10 g/dL was due to other causes such as myelodysplastic syndrome, bronchopleural fistula, Barrett’s esophagus, and Coumadin administration. The initial leukocyte level ranged from 3.2 to 13.2 × 109/L (median 5.7), and platelets ranged from 101 to 578 × 109/L (median 285.5). Only one patient had a serum creatinine > 2.0 mg/dL and it was unrelated to SWM. The serum monoclonal protein level at the time of diagnosis ranged from 1.5 to 5.2 g/dL (median 3.3); 12 (25%) were < 3 g/dL, while 10 (21%) were ≥4 g/dL. IgM kappa accounted for 75%, IgM lambda 21%, and 2 (4%) were biclonal. Immunofixation of the urine was positive in 97% (kappa 80%, lambda 17%, indeterminate 3%) of those tested. The size of the urine M protein ranged from unmeasurable to 1.4 g/24h (median 0.04 g/24h). Serum albumin ranged from 2.5 to 4.3 g/dL (median 3.6). Five patients (10%) had an albumin value < 3 g/dL. Ten (26%) had a reduction of 1 uninvolved immunoglobulin, while 8 (21%) had a reduction of both IgG and IgA immunoglobulins. Lymphoplasmacytic infiltration of the bone marrow ranged from 3% to 80%, (median 30%). Three (6%) had less than 10% infiltration, while 13 (27%) had 50% or greater infiltration. The 48 patients were followed for a total of 292 (range 0.5 to 22.6 years; median 3.7) person-years, during which time 33 progressed to symptomatic WM and one to AL amyloidosis. The expected number of WM based on the SEER data was 0.004 cases. Thus, relative risk of progression to WM was increased 7,586-fold (95% CI, 5,083 to 10,373). The median time to progression was 4.6 years. The absolute risk of progression to WM was 6% at 1 year, 39% at 3 years, and 55% at 5 years. The median survival after progression to symptomatic WM was 5.1 years. Seventy-three percent of patients died, indicating a robust follow-up. We evaluated sex, hemoglobin level, size of serum M protein, reduction of uninvolved immunoglobulins, presence of urinary monoclonal light chains, serum albumin level, and the proportion of lymphoplasmacytic infiltration of the bone marrow as risk factors for progression. Significant risk factors for progression with univariate analysis included size of the serum M protein, hemoglobin level, reduction in uninvolved immunoglobulins, and degree of bone marrow lymphoplasmacytic cell infiltration. Multivariate modeling revealed that the size of the M protein and the degree of bone marrow infiltration were the most important risk factors. We conclude that smoldering WM is a distinct clinical entity that needs to be differentiated from IgM MGUS and symptomatic Waldenstrom’s Macroglobulinemia.

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