48 Revisiting Hairy Cell Leukemia Variant and B-Prolymphocytic Leukemia in the Context of the WHO 5th Edition Classification

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Abstract Background The World Health Organization (WHO) Classification of Tumors 5th Edition introduced a new diagnostic category of splenic B-cell lymphoma/leukemia with prominent nucleoli (SBLPN) that encapsulates the entities of hairy cell leukemia variant (HCL-v) and B-prolymphocytic leukemia (B-PLL), previously recognized as distinct diagnoses. The new classification also describes prolymphocytic progression of chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) with greater clarity. Here we explore the implications of this change and its impacts on cases where a diagnosis of HCL-v and B-PLL was considered. Methods Cases from diagnoses made prior to the WHO 5th Edition (2005-2023) with prolymphocytic features or features suggestive of HCL-v were reviewed. The recorded results of morphology, immunophenotype, and genetic profiles were revisited. Pathology material revisiting is underway. Results The initial diagnostic specimens included 31 bone marrow, 3 spleen, 1 peripheral blood, and 3 lymph node specimens of 38 patients who often presented with prominent splenomegaly. The cohort included 31 males and 7 females in the age range of 44-85 years (median age: 71 years). Of the cases reviewed (Table 1), 12 (32%) were originally classified as HCL-v, and 11 (29%) were B-PLL. Additionally, the diagnosis of B-PLL or HCL-v were considered in cases that were favored to be atypical CLL/SLL (6 (16%)), splenic marginal zone lymphoma (SMZL; 4 (11%)), HCL (2 (5.3%)), and “splenic B-cell lymphoma, not further classifiable” (SBCL-NFC; 3 (7.9%)). Revisiting these cases using documented morphological, immunophenotypic and genetic characteristics, 16 (42%) were reclassified as SBPLN. The rest included prolymphocytic progression of CLL/SLL (8 (21%)), (S)MZL (6 (16%)), mantle cell lymphoma (MCL; 2 (5.3%)), CLL/SLL (1 (2.6%)), and SBCL-NFC (2 (5.3%)). Three cases (7.9%) did not have BRAF testing available for review; thus, the differential remained HCL vs SBLPN or SBCL-NFC. In many of these cases, subsequent investigations and evolving concepts had led to modifications of earlier diagnoses. Conclusion While HCL-v and B-PLL were originally classified as discrete pathologic entities, the WHO 5th Edition considers the features in these entities to be heterogeneous, and hence, the terms have been discontinued. An umbrella category or a placeholder category of SBLPN has been introduced. Additional research and considerations are required to dissect the true entities incorporated within SBLPN.

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