Notice that this gross specimen of a thymoma is about 5 cm in diameter. Size and mass are not typically useful criteria for the diagnosis of a thymoma due to the high variance in the size of the adult thymus. However, as a rough approximation, large size is highly suspicious of a thymoma especially in the adult. Remember that the thymus is typically only an atrophic wisp of tissue in the adult. Certainly any anterior mediastinal mass that is 5 cm in diameter is indicative of a thymoma.