B-cell chronic lymphocytic leukemia/small lymphocytic lymphoma involving bone marrow with an interfollicular pattern

B-cell chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) ma y involve the bone marrow in nodular, interstitial, diffuse, or mixed patte rns. However, B-cell CLL/SLL associated with large reactive germinal center s (the so-called interfollicular pattern) involving the bone marrow is not reported. We describe 2 examples of B-cell CLL/SLL that subtotally replaced the bone marrow with an interfollicular pattern. in both cases, the neopla sms were composed of small round lymphoid cells, proliferation centers also were present. The neoplasms surrounded large reactive germinal centers tha t were devoid of peripheral mantle zones. The germinal centers were paratra becular and nonparatrabecular in case 1 and nonparatrabecular in case 2. Fl ow cytometry immunophenotypic studies done on bone marrow aspiration sample s of both cases showed a uniform population of neoplastic cells positive fo r pan-B-cell antigens and the CD5 and CD23 antigens. Immunohistochemical st udies done on bone marrow biopsy sections supported the flow cytometry resu lts and demonstrated that the germinal centers were negative for BCL-2. B-c ell CLL/SLL may rarely involve the bone marrow with an interfollicular patt ern. Knowledge of this pattern will prevent confusion with follicle center lymphoma and large cell transformation, both of which initially were consid ered in the differential diagnosis of these cases.