Background and PurposeThe World Health Organization (WHO) has classified thymic carcinoma and other thymomas (types A, AB, and B) as different neoplasms. Myasthenia gravis (MG) is an early sign of thymoma and theoretically does not accompany thymic carcinoma; however, cases of thymic carcinoma with MG have been reported. Whether thymic carcinoma can accompany MG has yet to be established.MethodsThe medical records of patients who underwent thymectomy for MG between 1990 and 2011 in a single hospital were reviewed. All cases with the diagnostic code of "thymic carcinoma" or "thymoma type C" (old terminology) were selected. A pathologist re-reviewed the pathologic specimens using the new WHO criteria. The rate of thymic carcinoma among these MG patients was then calculated.ResultsA total of 81 patients with MG had thymic tumors, 10 of whom had thymic carcinomas or thymoma type C. Seven cases of well-differentiated thymic carcinomas (type B3) were excluded, leaving three (3.7%) cases of thymic carcinoma with MG. All three of these cases were type B3 thymoma with a focal squamous cell carcinoma component that was very small and well demarcated. In addition, two out of the three tumors were found to be at an early clinical stage. All of the cases survived without recurrence over follow-up periods of at least 5 years.ConclusionsThymic carcinoma transformation from thymoma can occur during the early stages of thymoma. The association of this condition with MG is not as rare as was previously thought. Thymic carcinomas accompanying MG had a predominant B3 thymoma component with a focal thymic carcinoma area (squamous cell carcinoma).