The cytologic findings of the tumor cells characteristic of the stages of thymomas were investigated to assess the invasiveness of the tumors. Forty-six patients with thymoma who underwent extensive thymectomy without pre-operative corticosteroid therapy were included in this study. The histologic subtypes included 18 round/oval, 20 mixed, and 8 spindle type. The stages of thymoma classified according to Masaoka's clinicopathological classification included 16 stage I, 20 stage II, 6 stage III, 2 stage IVa, and 2 stage IVb, and myasthenia gravis was recognized in 5 patients. Cytologic findings were retrospectively analyzed in the Papanicolaou-stained stamp smears obtained from the cut surfaces of thymoma specimens. Morphometry of the epithelial tumor cells using Cosmozone-1A was performed to evaluate the validity of our cytologic categories. Compared with the cytologic findings of stage I or II thymomas, those of epithelial tumor cells in stage III or IV more frequently showed necrotic background (50.0%-stage III or IV vs 11.1%-stage I or II, p=0.006), large clusters of epithelial tumor cells (70.0% vs 36.1%, p=0.055), marked nuclear enlargement (90.0% vs 52.7%, p=0.033), marked anisokaryosis (100% vs 52.7%, p=0.006), marked nuclear polymorphism (40.0% vs 5.5%, p=0.004), hyperchromasia (50.0% vs 11.4%, p=0.007) and prominent nucleoli (50.0% vs 16.6%, p=0.028) whereas no significant correlation was observed between cytologic findings and tumor volume. Morphometric studies of thymoma tumor cells revealed that the nuclear size (mean values, 78.8Eμm3-stage III or IV vs 58.2Eμm3-stage I or II), the coefficient of variation of the nuclear size (0.326 vs 0.282), and the nuclear rotundity (0.849 vs 0.858) differed significantly between the two categories (p<0.05). Our findings demonstrated that there were significant differences between the cytologic findings of epithelial tumor cells of stage I or II thymomas and those of stage III or IV thymomas, and that the cytologic findings of thymoma tumor cells appear to be useful for distinguishing between non-invasive and invasive thymomas.

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