A 62-year-old man has had fever and a 4-kg weight loss over the past 6 months. On physical examination, his temperature is 38.6deg C. He has generalized nontender lymphadenopathy, and the spleen tip is palpable. Laboratory studies show hemoglobin, 10.1 g/dL; hematocrit, 30.3%; platelet count, 140,000/mm3; and WBC count, 24,500/mm3 with 10% segmented neutrophils, 1% bands, 86% lymphocytes, and 3% monocytes. A cervical lymph node biopsy specimen microscopically shows a nodular pattern of small lymphoid cells. A bone marrow biopsy specimen shows infiltrates of similar small cells having surface immunoglobulins that are CD5+, but CD10-. Cytogenetic analysis indicates t (11;14) in these cells. What is the most likely diagnosis?

The immunophenotype is characteristic for mantle cell lymphoma. Of the lesions listed, lymphoblastic lymphoma and Burkitt lymphoma occur in a much younger age group. Burkitt lymphoma has a t (8;14) translocation. The remaining three lesions occur in an older age group. Of these, small lymphocytic lymphoma manifests with absolute lymphocytosis and the peripheral blood picture of chronic lymphocytic leukemia. Follicular lymphoma has a distinct and characteristic translocation t (14;18) involving the BCL2 gene. In contrast, mantle cell lymphoma, seen in older men, has the t (11;14) translocation, which activates the cyclin D1 (BCL1) gene; these tumors do not respond well to chemotherapy, particularly when it involves the peripheral blood.