Systemic, asymptomatic skeletal muscle infiltration by MALT lymphoma: Defining a 'silent infiltrator' phenotype with 18F-FDG PET/CT

We report a rare phenotype of Stage IV extranodal marginal zone lymphoma (MALT lymphoma) manifesting as diffuse, asymptomatic skeletal muscle and bone marrow infiltration in a 60-year-old female. Despite a high tumor burden and intense ¹⁸F-FDG avidity (SUVmax 8.8), muscle enzymes remained normal, indicating a non-destructive "silent infiltrator" growth pattern. The diagnosis was confirmed via biopsy (CD20+, CD5-, CyclinD1-) and negative MYD88 L265P mutation status, excluding lymphoplasmacytic lymphoma. The patient achieved a Complete Metabolic Response following Bendamustine-Rituximab therapy. This case underscores the utility of PET/CT in detecting occult systemic disease and defines a unique, indolent clinical variant of muscular MALT lymphoma.