Microthymoma: definition of the entity and distinction from nodular hyperplasia of the thymic epithelium (so-called microscopic thymoma).

We report 2 cases of microscopic-sized thymoma, which probably represents the earliest phase of thymoma development. The 2 patients presented with pure red cell aplasia and myasthenia gravis, respectively. The thymectomy specimens did not reveal tumor on gross examination, but histologically each contained small thymomas measuring 5 mm and 7 mm in largest dimension, respectively. One of the tumors was unencapsulated and involved a single lobule only, and the other was encapsulated and comprised two lobules. The tumors consisted of ovoid epithelial cells with pale nuclei and distinct nucleoli, scattered in a background of small lymphocytes. Foci of medullary differentiation and perivascular space were identified in the 2 cases, respectively. The lymphocytes were confirmed to be immature T cells on immunohistochemical studies (CD3+, TdT+). Except for the microscopic size, the morphology of the two tumors conforms to conventional type B1/B2 and type B2 thymoma, respectively. We propose calling such incidental small tumor "microthymoma" to distinguish it from the so-called microscopic thymoma, which is composed of small thymic epithelial nests and probably more appropriately termed "nodular hyperplasia" of the thymic epithelium.