The relationship between thymus and oncogenesis. A study of the incidence of non thymic malignancy in myasthenia gravis.

The records of 1243 patients with myasthenia gravis (M.G.) have been reviewed in a retrospective study of the incidence of extrathymic neoplasms. Ninety-four malignant neoplasms were traced.The onset of the disease (M.G.) coincided with a marked increase in the incidence of extrathymic neoplasms. The observed number of neoplasms in the year of onset of M.G. was three times higher than the expected in a control group. This was in sharp contrast to the lower than expected incidence in the years preceding the onset of M.G.The incidence remained at higher than the expected levels throughout the course of the disease in patients who did not undergo thymectomy, while in those patients who had thymectomy the incidence decreased to the levels of the general population after the second postoperative year.These observations suggest an oncogenic thymic influence. The possibility is discussed of the potential oncogenic role of abnormal clones of immunocompetent small lymphocytes of thymic origin.