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.