Objectives: Reports have been made of an altered rate of extrathymic malignancies in patients with myasthenia gravis (MG). This study compared the rate of such malignancies in a group of MG patients with an optimal control group.
Materials and methods: From the Norwegian Cause of Death Registry, we identified 249 dead MG patients (1951-2001) and a control group of 1,245 individuals (five per patient) dead in the same period, matched for sex and year of birth.
Results: Patients with MG had a lower occurrence of malignant disease as underlying or contributing cause of death than the controls (8.8% vs 27.2%, P < 0.001). The main difference was found for colorectal cancer, breast cancer and cancer in the upper digestive tract (esophagus and stomach).
Conclusions: We report a significantly lower rate of extrathymic malignancies in patients with MG than in controls, and we hypothesize that MG treatment or the immunological mechanisms involved in MG may protect patients with MG from developing an extrathymic malignancy.