Hairy cell leukemia presenting as a palpable breast mass

Abstract

Hairy cell leukemia (HCL) is a rare mature B cell malignancy that typically involves the bone marrow, peripheral blood, and spleen. Clinically detectable lymph node infiltration occurs in a minority of patients, and extranodal tissue involvement of sites such as the central nervous system, gastrointestinal tract, and skin is uncommon. Consequently, the vast majority of patients with HCL present with pancytopenia and splenomegaly but without lymphadenopathy or soft tissue masses. Herein we present a unique case of HCL presenting as a palpable breast mass without concomitant marrow or peripheral blood involvement and without radiologic evidence of other sites of disease. Several features of the tumor cell infiltrate (ample pale cytoplasm, nested appearance) evoked other entities that would more typically involve breast tissue such as the alveolar or solid variants of invasive lobular carcinoma. However, the immunophenotypic and molecular characteristics of the tumor cells were indistinguishable from classic HCL. The significant challenge in this case was recognizing this entity outside of its usual clinical and anatomic context. Although rare, HCL should be a diagnostic consideration in tumors arising at extramedullary/extranodal sites including the breast.