T‐cell lymphocytosis associated with lymphocyte‐rich thymoma

BACKGROUND Peripheral T‐cell lymphocytosis is found on very rare occasions in patients with thymomas. The immunophenotypic features and clonality of the lymphocytes in tumor and peripheral blood now are elucidating this enigmatic phenomenon. METHODS The author presents what is believed to be the seventh case of peripheral T‐cell lymphocytosis associated with thymoma and reviews the previous six cases. The pathology slides of the thymoma were reviewed with a pathologist who confirmed the presence of neoplastic thymic epithelium with cytokeratin stains. Immunophenotyping by flow cytometry was performed at Dianon Systems, Inc., on both the thymoma cells and cells in the peripheral blood. In addition, gene rearrangement analysis was performed on the peripheral lymphocytes using the previously described Southern blot analysis technique with immunoglobulin probes (heavy chain, kappa light chain, and lambda light chain) and T‐cell receptor gene probes (beta and gamma chains). RESULTS Analyses of the T cells within the thymoma and the peripheral blood confirm that the peripheral T cells are both polyclonal and more mature than those populating the thymoma. Clearly the peripheral T cells are not themselves neoplastic, and yet they represent more than physical "spillover" of the immature tumor‐related T cells. CONCLUSIONS Peripheral T‐cell lymphocytosis occurs rarely with locally aggressive, lymphocytic thymomas. Although it is clear that these cells are not neoplastic, as they are in other T‐cell proliferative disorders, the etiology of this unusual phenomenon remains obscure and may reflect the perturbation of systemic immunoregulation that accompanies thymic neoplasia. It is important to differentiate this condition from T‐cell lymphomas or leukemia to treat affected patients appropriately. Cancer 1997; 80:1409‐17. © 1997 American Cancer Society.