Thymoma and myasthenia gravis: a clinical study of 1,089 patients from Japan.

BACKGROUND The thymus or thymoma plays a role in the pathogenesis of myasthenia gravis (MG). Although previous studies have reported that the presence of MG in thymoma patients is an indicator of poor prognosis, recent reports have shown that the presence of MG no longer influences the prognosis or indicates a more favorable prognosis in thymoma patients. METHODS To clarify the difference in tumor behavior between thymoma with MG and that without MG, we compiled 1,089 thymomas treated between 1990 and 1994 from 115 institutes in Japan and analyzed the clinical characteristics of thymoma with MG compared with thymoma without MG. RESULTS MG was associated with 270 cases (24.8%) of thymoma. The total resection cases comprised 94.6% and 91.4%, and the recurrent cases comprised 6.4% and 8.3% in the groups with and without MG, respectively. The 5-year survival rates for thymoma with and without MG in stage III were 85.7% and 89.3%, respectively; and those in stage IV were 85.1% and 63.9%, respectively. The prognosis of thymoma with MG in stage IV tended to be better than that of thymoma without MG (p = 0.0523). The thymoma with MG group had fewer stage IVb thymomas than did the thymoma without MG group (15% vs 34%). The resectability rate in the thymoma with MG group was higher than that in the thymoma without MG group (60% vs 38%). CONCLUSIONS Thymoma with MG is equally or less malignant than that without MG. The infrequency of stage IVb thymomas and the high resectability rate had an influence on the favorable prognosis of stage IV thymoma with MG.