A multilobular variant of hairy cell leukemia with morphologic similarities to T-cell lymphoma.

Hairy cell leukemia is a distinct chronic lymphoproliferative disorder composed of morphologically unique B-lymphocytes. The diagnosis of hairy cell leukemia is usually based on the morphology of blood/bone marrow aspirate smears and bone marrow sections. We report three cases of hairy cell leukemia that had an unusual multilobated nuclear appearance seen on both smears and tissue sections. Many of the hairy cells exhibited marked nuclear lobulations and convolutions, giving rise to clover leaf-like nuclear configurations. The nuclear chromatin was finely reticular and characteristic of hairy cell leukemia. Bone marrow sections were hypercellular and the leukemic infiltrate was loosely distributed. A leukemic infiltrate was also seen in splenic sections in two cases in which splenectomy was carried out. Tartrate-resistant acid phosphatase cytochemistry was positive in all three cases. Flow cytometric analysis and paraffin section immunoperoxidase studies confirmed the B-cell lineage of the leukemic cells in all cases. The multilobulated nuclei described in these three cases are quite unusual in hairy cell leukemia; they could potentially lead to an erroneous diagnosis of T-cell lymphoma or leukemia. Features typical of hairy cell leukemia, such as the nuclear chromatin distribution and pattern of bone marrow infiltration, together with the clinical history, are helpful in establishing a correct diagnosis of these variant cases of hairy cell leukemia.