Dynamic serum IgM monitoring may refine response assessment in IgM-secreting marginal zone lymphoma: a two-center retrospective exploratory case series

We retrospectively evaluated whether dynamic serum IgM monitoring could complement PET/CT-based response assessment in 53 patients with IgM-secreting marginal zone lymphoma from two centers. A total of 112 paired IgM and PET/CT assessments were analyzed. Repeated-measures correlation showed that the percentage reduction in serum IgM was significantly associated with the percentage reduction in SUVmax (r rm = 0.65, P < 0.001). This association remained significant in a linear mixed-effects model adjusted for center, MZL subtype, treatment regimen, and baseline IgM level (β = 0.58, 95% CI 0.39–0.77, P < 0.001). Exploratory subgroup analysis suggested that IgM monitoring was more informative in patients with baseline IgM > 5 g/L, particularly > 10 g/L. At end-of-treatment assessment, PET/CT response and IgM response showed moderate agreement (85.7%; Cohen’s kappa = 0.56). Among patients achieving PET/CT and bone marrow negativity, those with persistent detectable M-protein had inferior 2-year progression-free survival compared with those achieving M-protein negativity (76.4% vs. 96.0%, P = 0.016). These findings suggest that serum IgM kinetics may serve as a complementary serologic marker for response assessment in IgM-secreting MZL, whereas persistent M-protein positivity after PET/CT negativity may indicate residual serologic disease burden and higher progression risk.