Objective: The influence of myasthenia gravis (MG) on risk of cancer is uncertain. Using nationwide, comprehensive data, we investigated the association between MG and occurrence of extrathymic malignancies in thymoma patients, and also assessed the risk of consecutive extrathymic malignancies after thymoma diagnosis.
Methods: Two hundred twelve thymoma patients were identified at the Cancer Registry of Norway between 1969 and 2005. Records on all extrathymic malignancies for these patients were supplied from the Registry's database. Comparisons were made between MG and non-MG patients and between thymoma patients and the general population.
Results: The frequency of extrathymic malignancies was similar in MG and non-MG thymoma patients, and so was the survival after thymoma diagnosis. Extrathymic malignancies occurred in 10% of thymoma patients within 10 years following the thymoma diagnosis. Thymoma patients had a significantly increased risk of developing an extrathymic malignancy compared to the general population. This was not linked to any specific kind of cancer. Thymoma morphology was not a significant predictor for an increased risk of consecutive cancer.
Conclusions: The immunological process underlying MG does not influence the risk of cancer in thymoma patients. Thymoma patients have a significantly increased risk of extrathymic malignancies. This is an intrinsic effect, being unaffected by a coexisting autoimmune disease such as MG and not specific for any type of cancer. Screening for extrathymic malignancies in thymoma patients is probably not recommendable, but clinicians should be aware of the high rate of extrathymic malignancies occurring in thymoma patients.