Aberrant Cytomorphological Features of Primary Plasma Cell Leukaemia: A Case Series

Abstract

Primary Plasma Cell Leukaemia (pPCL) is a rare but distinctive sub-type of Plasma Cell Myeloma (PCM) comprising approximately 2-4% of cases of PCM and characterised by short remissions and survivals. Apart from these clinical distinctions, this entity also demonstrates unique cytomorphological features. The authors here attempted to highlight this unusual spectrum of leukaemic plasma cells, posing diagnostic challenges in the blood and bone marrow examinations. The cases presented in this series (48 years old male, 72 years old male,68 years old male and 63 years old female) patients, showed presence of atypical cells in the peripheral smear with few cells portraying the characteristic “hairy cell” morphology, the fourth case in addition, showed small sized plasma cells resembling lymphocytes. Bone marrow biopsy in three cases showed haematopoetic cells replaced by sheets of atypical plasma cells. In addition, the fourth case showed atypical cells having convoluted and notched nuclei giving a differential diagnosis of lymphoplasmacytic lymphoma and small B-cell lymphoma with plasmacytic differentiation. As plasma cell neoplasm was not suspected clinically in all four cases, flow cytometry was sought for confirmation which showed atypical cells expressing CD38 (+), CD138 (+), CD56 (–) with Cykappa (+) in first case, Cylambda (+) in the second case and Cykappa (+) in the fourth case with aberrant expression of cyclin D1 thus confirming a diagnosis of PCL in first two cases and PCM-lymphocytic type with PCL in the fourth case. Immunophenotyping was not performed in the third case due to financial constraints. This case series showcases this mélange of cytomorphological variations of plasma cells ranging from cells masquerading as hairy cells to small sized plasma cells resembling lymphocytes, each posing a unique diagnostic dilemma. This has been rarely discussed in prior published case series. Aberrant morphological features rarely seen in PCL, could lead the pathologist astray from the diagnosis, therefore appropriate use of ancillary tests are essential to arrive at a correct diagnosis.