Pb2283: in the era of immunophenotyping and molecular diagnostics does morphology play an essential role in differentiating hairy cell leukemia from its mimics?

Topic: 18. Indolent and mantle-cell non-Hodgkin lymphoma - Clinical Background: Hairy cell leukemia (HCL) accounts for 2% of all lymphoid leukemias. It is characterized by splenomegaly, cytopenias and atypical lymphoid cells with circumferential cytoplasmic projections. While HCL responds well to chemotherapy, the other morphological differential diagnosis such as hairy cell leukemia variant (HCL-v) and splenic marginal zone lymphoma (SMZL) do not respond well to HCL therapy, and have a significantly lower survival. Distinguishing HCL from HCLv and SMZL can be challenging as clinical and pathologic findings often overlap. Aims: To analyse the clinical, morphological and immunophenotypic features of HCL, HCL-v and SMZL and to evaluate the role of morphological features in differentiating these entities. Methods: This was an observational study which included eight cases of HCL and its mimics diagnosed over a 4 and a half year period. The clinical details, and complete blood counts were recorded. The peripheral smear (PS) and bone marrow aspirate (BMA) slides were reviewed for morphological features such as cytoplasmic projections, amount of cytoplasm, nuclear shape and nucleoli. Bone marrow biopsy (BMB) slides were reviewed for cellularity, pattern and percentage of marrow infiltration, and residual hematopoiesis. Flow cytometry was done in 3 cases which confirmed the diagnosis. Immunohistochemistry (IHC) panel of CD20, CD3, CD5, CD10, Cyclin D1, CD11c, CD25, CD103, CD123 and Annexin A 1 was done in other cases. Results: There were 5 cases of HCL, 2 cases of HCLv and 1 case of SMZL. All cases of HCL and SMZL had pancytopenia while HCLv had normal or increased total counts. Cytoplasmic projections were seen in 60% of HCL, and 100% of HClv. Prominent nucleoli were seen in HCLv (100%) but not in HCL. HCL showed significantly greater marrow infiltration over HCL-v. Marrow was hypocellular in 80% of HCL. On immunophenotyping (IPT), HCL expressed CD19, CD11c, CD25, CD103 and CD123 whereas HCLv lacked CD25. Annexin was positive in all cases of HCL while negative in HCLv and SLVL. (Table 1). Summary/Conclusion: This case series highlights the importance of identifying the subtle morphological features on peripheral smear, BMA/BMB that help in diagnosing HCL and its mimics. The ability to narrow down the IHC panel based on morphology will be helpful in limited resource settings. Table 1: Morphological findings in peripheral blood and bone marrow in HCL, HCLv and SMZL.Keywords: Hairy cell leukemia, Immunophenotype, Splenic marginal zone lymphoma