Biclonal plasma cell neoplasms (PCNs) are exceedingly rare, representing fewer than 5% of multiple myeloma cases. They are characterized by the presence of two distinct monoclonal immunoglobulin populations, either from independent clones or from a single clone with phenotypic diversity. Accurate recognition of biclonality is essential for diagnostic and prognostic assessment.

We report a rare case of biclonal plasma cell neoplasm (PCN) in an 82-year-old female admitted with acute kidney injury. Serum immunofixation demonstrated a biclonal gammopathy with both kappa and lambda light chains. Bone marrow flow cytometry revealed two CD19- negative plasma cell populations, both expressing strong CD38 and CD138. Population A (2.7%) showed increased forward and side scatter, kappa light chain restriction, and strong expression of CD56 and CD200. Population B (0.5%) demonstrated decreased scatter properties, lambda restriction, and weak CD56 and CD200 expression. Morphologic analysis using May-Grünwald Giemsa staining identified two distinct plasma cell morphologies: one composed of larger cells with intensely eosinophilic, flame-like cytoplasm, typically associated with IgA or kappa-producing plasmocytes; the other consisting of smaller cells with basophilic cytoplasm, dense chromatin, and eccentric nuclei, consistent with lambda-restricted plasma cells. FISH analysis showed three copies of the TP53 gene, a high-risk cytogenetic alteration. These findings support the coexistence of two phenotypically and morphologically distinct clonal plasma cell populations within the same marrow, consistent with a true biclonal PCN.

Biclonal PCNs are uncommon and may reflect clonal evolution or synchronous transformation of distinct progenitor cells. The coexistence of flame-like and basophilic plasma cell morphologies, coupled with divergent immunophenotypes and light chain restriction, suggests true biclonality. High-risk cytogenetic alterations such as TP53 gain further contribute to an aggressive disease course, as noted in previous reports (Gentry et al., 2013; Haidary et al., 2022)