Plasma Cell Myeloma Mimicking Metastatic Carcinoma with CD138 positivity: a Potential Diagnostic Pitfall

Background: Plasma cell myeloma is a kind of multifocal proliferation of neoplastic plasma cells in bone marrow. Morphology of myeloma plasma cells varies from mature to immature form, plasmablastic, and pleomorphic cells, with the proportion of plasma cells changing from a slight increase to > 90%. Several morphologic variants of PCM have been reported. Methods: Herein, we present a rare case of PCM with typical morphological features of bone marrow metastatic carcinoma, association with CD138 positivity, and a complex karyotype. Results: The diagnosis of PCM was made based on a combination of the clinical features, morphology, immunofix-ation electrophoresis, flow cytometry immunophenotyping, and bone marrow biopsy. Overall, the result was in accord with PCM based on the WHO classification. Conclusions: The case focuses on the wide morphological variants of PCM and highlights the reason why PCM should be taken as a differential diagnosis when one presents with typical morphological feature and common an-tigens expression of bone marrow metastatic carcinoma. (Clin. Lab. 2022;68:xx-xx. DOI: 10.7754/Clin.Lab.2021.210635)