A rare case of nodal hairy cell leukemia exhibiting characteristic morphological and immunohistochemical features

A 51-year-old woman presented with fatigue and bruising. The workup revealed neutropenia, monocytopenia, and thrombocytopenia. A bone marrow (BM) biopsy confirmed a diagnosis of hairy cell leukemia (HCL). Flow cytometry identified a λ-restricted B-cell population positive for CD103, CD25, and CD11c. Molecular analysis revealed a BRAF p.V600E mutation. Later, she developed splenomegaly and lymphadenopathy. Lymph node (LN) biopsy revealed abnormal lymphoid infiltrates with nodular appearance around sinusoids. The infiltrating cells had abundant pale cytoplasm (panel A, hematoxylin and eosin [H&E] staining, original magnification ×20; panel B, H&E staining, original magnification ×60) and positive immunohistochemistry (IHC) for CD20 (panel C, original magnification ×40), cyclin D1 (weak) (panel D, original magnification ×40), BRAF V600E (panel E, original magnification × 40), and annexin (panel F, original magnification ×40).

HCL typically affects the BM, blood, and spleen, while rarely involving LNs. Distinguishing HCL from other B-cell lymphomas can be challenging due to overlapping histology and IHC profiles. For example, HCL and nodal marginal zone lymphoma (MZL) can exhibit nodular patterns with small to medium-sized cells with pale cytoplasm. In our case, the neoplastic cells exhibited nodular infiltration around the sinusoids, whereas in nodal MZL, these cells surround reactive secondary follicles. Although HCL and mantle cell lymphoma (MCL) express cyclin D1, HCL shows weak and variable expression and MCL shows strong expression and has a CCND1-IGH translocation (absent in HCL). In our case, history of HCL and distinct growth pattern prompted confirmatory IHC testing. In summary, careful histological examination and specific ancillary studies confirmed this case of nodal HCL.

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