Thymoma: a study of the pathologic classification of 71 cases with evaluation of the Muller-Hermelink system

A pathologic study of 71 consecutive cases of thymoma was conducted to determine the value of histologic classification for predicting the invasive potential of thymomas. The traditional and newly proposed Muller-Hermelink classification systems were studied for correlation with thymomas of the three clinicopathologic stages identified as noninvasive thymoma, microinvasive thymoma, and macroinvasive thymoma. None of the histologic types classified by the traditional system were correlated specifically with invasive growth. In contrast, the cortical-type thymoma of the Muller-Hermelink system correlated strongly with both microinvasive thymoma and macroinvasive thymoma (P < .001 for both). Although not all cortical thymomas manifested invasive growth and not all invasive thymomas were of the cortical type, the statistical data suggested that cortical thymomas should be regarded as potentially malignant tumors with the propensity for invasive growth. Spindle-cell thymoma of the traditional system or medullary thymoma of the Muller-Hermelink system, on the other hand, was not associated with invasiveness and behaved as a benign tumor. All other thymomas that were not purely spindle cell or medullary cell in nature had a potential for invasion and aggressive behavior, and this potential seemed to be inversely proportional to the percentage of fusiform cells in such neoplasms. Nevertheless, the results of this study support the usefulness of the Muller-Hermelink classification system in predicting the aggressive potential of thymomas. A group of microinvasive thymomas was identified as a separate group for analysis in this study. Although no recurrences were observed during the follow-up period, microinvasive thymomas deserve further study as a separate group because they correlated strongly with cortical thymoma. The association of thymoma with myasthenia gravis was rather high (63.4%) in this study. Both the mixed-type thymoma of the traditional classification system and the cortical-type thymoma of the Muller-Hermelink system showed statistically significant correlation with myasthenia gravis (P = .002 and .001, respectively).