Differential diagnosis between undifferentiated tumor and thymoma by electron microscopy and immunohistochemical labelling.

This study of a particular case of tumor posed and resolved problems of differential diagnosis between an undifferentiated tumor and a thymoma by using electron microscopy in association with immunocytochemical methods. The first step was the distinction between an epithelial and a mesenchymal tumor, which was done by electron microscopy and immunofluorescence observation with anti-keratin antibody. The second step, a new approach to this problem, was the distinction between an epithelial tumor of thymic origin and another tumor located in the mediastinal lodge. A clear distinction was made by observation in immunofluorescence using anti-thymulin monoclonal antibody. This double approach permits differential diagnosis, excludes neoplasms of germ-cell origin, malignant lymphomas and leukemias, as well as mesenchymal tumors, and affirms the thymic origin of the tumor observed. A second type of cell observed in this tumor with a peculiar aspect, different from all types of epithelial cells observed in normal thymus, is discussed.