Hairy cell leukemia presenting with a lytic bone lesion

Hairy cell leukemia (HCL) is an indolent neoplasm of small mature B lymphoid cells with characteristic morphologic features usually involving the peripheral blood, bone marrow, and spleen. It constitutes approximately 2% of adult leukemias and has a male predominance. Patients usually present with weakness, splenomegaly, and pancytopenia. The overall prognosis is favorable with appropriate treatment including purine analogs, interferon-alpha, and radiation. We report a patient with a history of breast cancer who presented with a left femoral lytic bone lesion that was subsequently diagnosed as HCL by morphology, immunohistochemistry, flow cytometry, and molecular genetic techniques. It was important to biopsy the lesion to establish the correct diagnosis, as HCL is a highly treatment-responsive malignancy.