Lymphomatous variant of hairy cell leukaemia: A distinctive presentation mimicking low-grade B-cell lymphoma

Aims: Hairy cell leukaemia (HCL) is an indolent B-cell neoplasm that primarily involves the peripheral blood, bone marrow, and spleen. Rarely, patients with HCL present with a lymphoma-like clinicopathological picture mimicking other types of low-grade B-cell lymphoma. Distinguishing HCL from other types of lymphoma is essential, given the different treatments and prognoses. We report two cases of the lymphomatous variant of HCL to draw attention to this unusual presentation. Methods and results: Two cases of HCL presented initially as salivary gland and soft tissue masses. Morphologically, the neoplastic cells showed a typical fried-egg or monocytoid appearance with reniform nuclei, and were positive for CD11c, CD20, CD25, CD103, annexin A1, and cyclin D1. Both cases were positive for the BRAF V600E mutation, as shown by pyrosequencing and mutation-specific immunohistochemistry. The diagnosis was challenging in both patients, and one patient experienced 5 years of disease relapse and progression after treatment with multiple lymphoma-type regimens before the final diagnosis of HCL was established and complete remission was achieved following cladribine therapy. Conclusions: HCL can very rarely present as an extranodal mass. Being aware of this unusual presentation of HCL is essential to avoid diagnostic confusion with other types of low-grade B-cell lymphoma and to facilitate appropriate therapy.