DNA flow cytometric study of thymic epithelial tumors with evaluation of its usefulness in the pathologic classification

A DNA flow cytometric study was performed on 64 thymic epithelial tumors using the modified Hedley method with formalin-fixed, paraffin-embedded tissues to evaluate whether ploidy analysis can be a useful aid in differentiating invasive thymoma (IT) from non-invasive thymoma (NT) and in understanding the ploidy pattern of various histologic types of thymic carcinomas (TCs). The IT group was further subdivided into macroinvasive thymoma (macro-IT) and microinvasive thymoma (micro-IT). Six cases were excluded due to a coefficient of variation greater than 6. Aneuploidy with a DNA index greater than 1.05 was found in every group: two of 16 NTs, two of nine micro-ITs, 11 of 13 macro-ITs, and 12 of 20 TCs were aneuploid. However, the proportions of aneuploidy of macro-IT and TC were statistically significantly higher than those of NT and micro-IT (P < .001). Similarly, the DNA indexes of macro-IT and TC were significantly higher than those of NT and micro-IT (P < .004). Therefore, micro-ITs were cytometrically similar to NTs and could be grouped with the NTs. Although ploidy analysis cannot predict whether an individual thymoma is invasive, aneuploidy and a high DNA index would favor a macro-IT, which also has a significantly higher S-phase fraction. Thymic carcinoma and IT cannot be differentiated by DNA flow cytometry. No definite ploidy pattern was observed for various histologic types of TC, but transition from diploid to aneuploid was demonstrated in thymomas undergoing malignant transformation into TC.