Extrathymic malignancies in patients with myasthenia gravis: A Japanese multicenter study.

Objective Although various studies have investigated the association between myasthenia gravis (MG) and malignancies, a clear consensus has not yet been reached. Therefore, this study investigated the relationship between MG and malignancy in patients who developed malignancies. Methods We collected clinical data from the Japan MG Registry 2021 survey, conducted between April and October 2021. The presence of thymoma, MG severity, and extrathymic malignant complications was examined. For patients with concomitant malignancies, the malignancy type, age at onset of malignancy, and use of immune checkpoint inhibitors were assessed. Cox proportional hazards models were used to identify the predictors of malignancy. Results Of the 1690 patients with MG, 115 (6.8%) had malignancies. A multivariable analysis revealed that age at MG onset (hazard ratio [HR] 1.050, 95% confidence interval [CI] 1.037-1.063, P <0.001) and thymoma presence (HR 1.656, 95% CI 1.123-2.444, P = 0.009) were significantly and positively associated with the occurrence of extrathymic malignancies. Conclusion Thymomas and an older MG onset are risk factors for extrathymic malignancies in patients with MG. The development of malignancy should be carefully monitored, particularly in MG patients with thymoma.