Thymic epithelial tumours: evaluation of malignant grade by quantification of proliferating cell nuclear antigen and nucleolar organizer regions.

Cellular proliferation was studied by quantification of proliferating cell nuclear antigen (PCNA) and argyrophilic nucleolar organizer regions (AgNORs) of cells in 29 thymic epithelial tumours: 8 noninvasive (7 cortical and 1 mixed) thymomas, 11 invasive/metastatic (all cortical) thymomas, and 10 thymic carcinomas. Thymic carcinoma showed the highest percentage of cells positive for PCNA (14.73 +/- 5.419%) and the largest mean number of AgNORs per nucleus (4.89 +/- 0.756). The mean percentage of PCNA-positive cells and number of AgNORs in thymoma groups were as follows: in noninvasive thymoma 2.96 +/- 1.256% and 2.73 +/- 0.647, respectively, and in invasive/metastatic thymoma 4.41 +/- 1.823% and 3.68 +/- 1.148, respectively. The differences in PCNA and AgNORs were statistically significant between thymic carcinoma and each of thymoma groups. The overlap of the values between these tumours was minimal in the PCNA stains, although it was considerable in AgNOR counts as previously noted. However, there was no statistically significant difference in these markers between noninvasive and invasive/metastatic thymomas. These results indicate that thymoma in general is a slow-growing tumour compared with thymic carcinoma and that noninvasive thymoma is similar to invasive/metastatic thymoma with regard to proliferative activity; these latter two tumours may represent an essentially identical type in different stages of progression.