Lymphoplasmacytic Lymphoma Mimicking Plasma Cell Neoplasm: A Rare Case Report

Lymphoplasmacytic Lymphoma (LPL) is an uncommon B-cell neoplasm that may closely mimic plasma cell disorders, leading to diagnostic uncertainty. We report the case of a 56-year-old man who presented with fatigue and bone pain. Bone marrow evaluation revealed an admixture of plasma cells and plasmacytoid lymphocytes, showing significant morphological overlap between LPL and plasma cell neoplasm. An extended Immunohistochemical (IHC) panel demonstrated CD20 and CD79a positivity in B-lymphoid cells, CD138 positivity in plasma cells, and kappa light-chain restriction, consistent with a clonal B-cell proliferation with plasmacytic differentiation. Correlation of clinical, radiological, and laboratory findings established the final diagnosis of LPL. This case emphasises the limitations of relying solely on morphology and highlights the essential role of immunophenotyping in differentiating LPL from plasma cell neoplasms. The distinction carries therapeutic and prognostic significance; LPL typically responds to rituximab-based regimens targeting B cells, whereas plasma cell neoplasms require anti-myeloma therapy. Recognising such atypical marrow presentations is crucial to prevent misclassification, enable appropriate management, and improve patient outcomes.