Ultrastructural studies of an acth-secreting thymic tumor

An ACTH-secreting thymoma is reported in a 26-year-old woman with Cushing's syndrome. The tumor was epithelial in type as seen with the light microscope and resembled other thymomas in patients with Cushing's disease reported in the literature. Ultramicroscopically, the epithelial nature of the tumor was confirmed. Neurosecretory granules were easily identified in the tumor cells. These granule-containing cells were similar to the tumor cells of oat cell carcinomas of the lung and bronchial carcinoids. The granules were almost identical to those seen ultrastructurally in another asymptomatic thymoma reported elsewhere. It is concluded that the granules may be the source of the ACTH production, but tumors that harbor them may not be necessarily associated with Cushing's syndrome. In the case of the oat cell carcinoma of the lung, the rapid death of the patient may preclude the development of the fullblown syndrome.