Pii: S1010-7940(00)00523-6

Objective: There have been con ̄icts concerning the criteria for diagnosing malignant epithelial neoplasms of thymic origin. To differentiate thymic carcinomas from thymomas, the maturation stage of T-lineage lymphoid cells in®ltrating thymomas and thymic carcinomas was examined by ̄ow cytometry to associate it with the degree of tumor malignancy. Methods: Multidimensional ̄ow cytometric analysis was performed on the lymphoid cells extracted from 27 thymic epithelial neoplasms (14 encapsulated thymomas, ten invasive thymomas, and three thymic carcinomas) by using anti-CD3, -CD4, -CD8, -CD10, -CD20, -CD38, -CD45RA, and -CD45RO monoclonal antibodies. Results: CD4 and CD8 were co-expressed on 76.8% of the lymphoid cells in encapsulated thymoma (N ˆ 14), 59.2% in invasive thymoma (N ˆ 10), and 6.7% in thymic cancer (N ˆ 3). The percentage of CD4or CD8single positive cells was 11.4% in encapsulated thymoma, 23.9% in invasive thymoma, and 77.7% in thymic cancer. The percentage of CD10-positive cells was 20.8% in encapsulated thymoma, 13.2% in invasive thymoma, and 6.0% in thymic cancer. The percentage of CD20-positive cell was 2.6% in encapsulated thymoma, 3.3% in invasive thymoma, and 31.6% in thymic cancer. There were signi®cant statistical differences in the percentages of CD4/CD8 double positive cells, CD4or CD8-single positive cells, CD10-positive cells and CD20-positive cells among the three groups. Two cases classi®ed as invasive thymoma by pathohistological examination, however, showed the in®ltration of mature lymphocytes like as thymic cancers. Conclusions: CD41CD81 or CD101 T-lineage cells were the most reliable markers of the benignancy of thymic epithelial tumors. CD4or CD8-single positive cells or CD20-positive cells were characteristic in thymic carcinoma. Flow cytometry on the maturity of lymphoid cells in®ltrating thymic epithelial tumors was feasible for determining their degree of malignancy. Some invasive thymomas showed the intermediate characteristics with thymomatous epithelia and mature lymphoid cells. q 2000 Elsevier Science B.V. All rights reserved.