Degree of malignancy of thymic epithelial tumors in terms of nuclear DNA content and nuclear area. An analysis of 39 cases.

For determination of the degree of malignancy among thymic epithelial tumors, the DNA content and area of nuclei in 13 cases each of noninvasive thymoma, invasive thymoma, and thymic carcinoma were investigated by cytofluorometry and morphometry. The nuclear DNA content was determined in terms of the mean nuclear DNA content, DNA histogram pattern, and the occurrence of the aneuploid stem cell line. The mean nuclear DNA content of the thymic carcinoma was significantly higher than that of both subgroups of thymoma (P less than 0.01), but there was no significant difference between noninvasive and invasive thymomas. The aneuploid stem cell line appeared in 92.3% of thymic carcinomas, one case (7.7%) of invasive thymomas, and none of noninvasive thymomas. Abnormal DNA histogram patterns were seen in 53.8% of thymic carcinomas and none of the thymomas. The mean nuclear area increased significantly in the increasing order of noninvasive thymoma, invasive thymoma, and thymic carcinoma (P less than 0.01). The cytofluorometric and morphometric results demonstrated a significant difference in degree of malignancy between thymic carcinoma and thymoma; however, there was a trend toward an increasing degree of malignancy from noninvasive to invasive thymomas, yet there was a sizeable overlap in results between the two groups. Therefore, these two methods are not satisfactory for predicting the behavior of an individual case of noninvasive or invasive thymoma.