Risk of extrathymic cancer in patients with myasthenia gravis in Taiwan: a nationwide population-based study

Background and purpose: The relationship between myasthenia gravis (MG) and extrathymic malignancies has not been determined. This study aimed to explore the risk of extrathymic malignancy in patients with MG based on a nationwide population-based dataset. Methods: We identified 2614 patients with MG from the Taiwan National Health Insurance database between 1997 and 2005 and compared the incidence rates of extrathymic malignancies with 15 684 randomly selected age-, sex-, and comorbidity-matched subjects without MG. Both cohorts were followed until the end of 2009. Cox proportional hazard model was used to evaluate the predictors of extrathymic malignancy in the MG cohort, including age, sex, comorbidities, and prescription drugs. Results: After an average follow-up of 8 years, the MG cohort had a higher risk of extrathymic cancers with an incidence rate ratio (IRR) of 1.38 (95% CI 1.12-1.68, P = 0.002) than the control cohort. Although breast cancer was the most common cancer found, no statistically significant relationship between MG and any specific malignancy was observed. Cox multivariate proportional hazards analysis showed that only age (HR = 1.05, 95% CI 1.04-1.06, P < 0.001) and liver cirrhosis (IRR = 3.85, 95% CI 1.22-12.14, P = 0.021) were predictors of extrathymic cancers in the MG cohort. Conclusions: Our study showed that patients with MG had an increased risk of extrathymic malignancy in a follow-up of 8 years, but no specific susceptibility to certain malignancies was found.