[Grading of malignancy of thymomas by flow cytometry]

The nuclear DNA contents of paraffin embedded archival tissues from 33 cases of thymoma and carcinoma of the thymus were studied by flow cytometry (FCM). Twelve cases were ruled out because of coefficients of variance (CV) > 10.0, leaving 21 cases for analysis. Included were 9 cases of non-invasive thymomas and 12 cases of invasive thymomas and thymic carcinomas. Among non-invasive thymomas, the DNA content was diploid in 4 cases and aneuploid in 5 cases. The DNA content was aneuploid in all 12 cases of invasive thymomas and thymic carcinomas. Proliferative activity, determined by measuring S phase percentage and proliferative index, was significantly increased in invasive thymomas and thymic carcinomas as compared with the non-invasive ones. The average S phase percentage for invasive thymomas and thymic carcinomas was 29.17 +/- 6.91, and for non-invasive thymomas it was 15.86 +/- 3.17 (P < 0.001). The average proliferative indices in the invasive and non-invasive groups were 47.76 +/- 5.03 and 32.58 +/- 6.58, respectively (P < 0.001).