Background Thymoma patients with no prior history of myasthenia gravis (MG) occasionally have MG after thymectomy. This study aimed to identify risk factors for developing postthymectomy MG. Methods We retrospectively reviewed the characteristics and clinical outcomes of thymoma patients without preoperative MG who underwent a thymectomy at our institute. Results Of the 229 thymoma patients who underwent surgery from 1991 to 2011, 123 had no history of MG. The serum of all patients was analyzed for acetylcholine receptor antibodies (AchR-Ab), and 33 patients had detectable levels (greater than 0.2 nmol/L). Ten seropositive patients had MG develop postoperatively within 3 to 2,859 days, whereas 90 seronegative patients did not. Univariate analysis showed that a positive result for serum AchR-Ab (p < 0.001), type B1/B2/B3 thymoma histology (p = 0.025), and incomplete resection (p = 0.008) were risk factors for the development of MG. In patients with post-thymectomy MG, the AchR-Ab levels at the onset of MG were significantly higher than the preoperative levels (p = 0.036). The analysis of the 33 antibody-positive patients showed a significant correlation between antibody levels and the onset of disease (p = 0.003). The analysis of 12 patients with incomplete resection revealed that perioperative chemotherapy or radiotherapy reduced the risk of development of MG (p = 0.009). Conclusions A positive result for serum AchR-Ab, type B1/B2/B3 thymoma histology, and incomplete resection were considered risk factors for the development of postthymectomy MG. Preoperative levels of serum AchR-Ab should be examined in all thymoma patients. Perioperative chemotherapy or radiotherapy for the patients with incomplete resection might reduce the risk of MG development.
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