Flow-cytometric diagnosis of thymoma using needle biopsy specimens

Thymoma is a neoplasm of thymic epithelial cells which is characteristically associated with a large number of non-neoplastic T lymphocytes. These T cells are induced by epithelial cells and show a unique phenotype of CD4(+)8(+) double positive (DP), when studied by flow cytometry. This DP phenotype can be used as one of the diagnostic criteria to indicate a thymoma. The preoperative diagnosis of thymoma and other thymic tumors is an important problem because the treatment differs according to the diagnosis.

A flow-cytometric analysis of needle biopsy specimens was performed on ten thymic tumors. The results were then compared with the findings of a histological diagnosis using conventional hematoxylin-eosin (H&E) staining.

Six cases with high frequencies of DP cells were diagnosed as thymoma by a flow-cytometric analysis, and were confirmed by a histological analysis of the resected specimen. Flow cytometry did not suggest a thymoma in the other four cases with few DP cells. The final diagnoses of the resected specimen of these four cases were: one thymic carcinoma, one malignant lymphoma, and two germ-cell tumors. The accuracy and specificity of diagnosis of thymoma using a fluorescence-activated cell sorting analysis from needle biopsy specimens were 6/6 (100%). On the other hand, the specificity of H&E staining from needle biopsy specimens for the diagnosis of thymoma was 6/6 (100%), but the accuracy was only 6/9 (66.7%).

Flow cytometry can be applied to needle biopsy specimens and thus is considered to offer useful information for the preoperative diagnosis of thymic tumors.