MICROSCOPIC THYMOMA - HISTOLOGICAL EVIDENCE OF MULTIFOCAL CORTICAL AND MEDULLARY ORIGIN

Twenty cases of macroscopically non-neoplastic thymuses obtained from patients with myasthenia gravis have been studied histologically. Seve n cases were characterized by lymphoid follicular hyperplasia and 13 b y involutional changes of variable degree. In three cases (15%), one w ith lymphoid follicular hyperplasia and two with involutional changes, multiple microscopic epithelial lesions, 0.2-0.4 mm in diameter and c onsistent with foci of microscopic thymoma, were observed. Most of the m were related to the thymic cortex and one, displaying a different hi stological pattern, was located in a medullary area. These observation s provide morphological evidence of a possible multifocal origin of th ymoma from distinct epithelial clones present in the different topogra phic areas of the human thymus.