Delta‐T‐lymphocytosis in a patient with thymoma

Background. Malignant thymoma is composed of neoplastic epithelial cells and small lymphocytes. Rarely, patients also may have peripheral T‐lymphocytosis. These lymphocytes have been considered nonneoplastic because of their microscopic appearance and immunophenotype, as well as gene rearrangement studies. Methods. A 42‐year‐old man developed lymphocytosis 3 years after the completion of intensive combined chemoradiotherapy protocol for lymphocytic thymoma. These peripheral blood lymphocytes were evaluated phenotypically and genotypically. Results. Immunophenotyping established that the cells were CD3 positive, CD4 negative, CD8 negative, T‐cell receptor (TCR)‐alpha/beta negative, and TCR‐gamma/delta positive. Gene rearrangement studies with TCR‐delta probe confirmed the monoclonality of these cells. Chromosome analysis showed deletion of chromosome Y. The clinical course was progressive and had the features of malignant lymphoma. Conclusions. To the authors' knowledge, this is the first report of a patient with thymoma in whom monoclonal proliferation of T‐gamma/delta peripheral blood lymphocytes was confirmed immunophenotypically and genotypically. These monoclonal TCR‐gamma/delta lymphocytes may belong to the malignant clone of the thymoma; however, the possibility that they represent an evolution of a second lymphatic malignancy cannot be excluded.