The Effect of Thymectomy on the Incidence of Extrathymic Cancers and Autoimmune Diseases in Patients With Non‐Thymomatous Myasthenia Gravis

INTRODUCTION/AIMS: Thymectomy is associated with positive effects on myasthenia gravis (MG), but there is conflicting evidence regarding potential deleterious long-term outcomes, such as increased cancer and autoimmune disease risk. We aimed to assess these outcomes in thymectomized versus non-thymectomized MG patients. METHODS: We retrospectively reviewed the medical records of patients with acquired, autoimmune MG followed at the Prosserman Neuromuscular Clinic, University Health Network (UHN), between January 2000 and July 2025. We excluded patients with malignancy or other autoimmune diseases before MG. We created matched datasets (thymectomy vs. no-thymectomy) and built Cox models to assess the rate of new malignancy or autoimmune disease. We also used inverse probability of treatment weights (IPTW) to balance relevant covariates (age at MG onset, sex, disease duration, medications) in the full sample, and tested Cox models in the weighted dataset for each outcome. RESULTS: Of 566 patients, 106 had a thymoma and were excluded. Of the 460 patients included, 161 had a thymectomy. There was no difference in the hazard ratio (HR) for a new malignancy in the matched cohort (HR: 0.94, CI 0.83-1.07) nor in the IPTW model. The incidence of autoimmune disease was slightly higher in the thymectomy group in the matched cohort (HR 1.33 [1.15-1.52]), but there was no difference in the IPTW analyses (HR 1.55, CI [0.40-5.9]). DISCUSSION: In this cohort of patients with non-thymomatous MG, thymectomy was not associated with an increased risk of malignancy. Different models for a second autoimmune disease had conflicting findings, requiring further research.