Application and pitfalls of immunophenotyping in challenging plasma cell neoplasms: A case series.

Multiple myeloma (MM) is an incurable malignant plasma cell neoplasm, representing the second most common hematopoietic cancer. As plasma cell neoplasms are clonal and often secrete a monoclonal protein (M-spike), laboratory diagnosis is usually straightforward, especially when ancillary studies such as immunohistochemistry, flow cytometry, and protein electrophoresis are available in addition to microscopic examination. Despite the repertoire of diagnostic tools, rare cases pose diagnostic dilemmas, especially when reagent antibodies do not react as expected, extent of disease is patchy, or when disease occurs in unique age groups. In this retrospective study, we report a series of challenging diagnostic cases, discussing aberrant findings and comparing them to more classic counterparts. Twelve cases collected during routine clinical sign-out were reanalyzed and include examples of MGUS, classic multiple myeloma, t(11; 14) rearranged myeloma, minimal residual disease, AA and AL amyloidosis, truncated light chain, non-secretory and non-producer myeloma, biphenotypic myeloma, oligoclonal expansion after bone marrow transplant, and plasma cell leukemia in a young adult. This cohort showcases the diversity of atypical presentations of plasma cell neoplasms, and we highlight standardized approaches to workup to avoid diagnostic pitfalls.