Postoperative photomicrographs of lesion in thymoma component. (a) The neoplastic cells were large and polygonal with large nuclei and prominent central nucleoli in the lymphocyte-rich background. (b) Some small epidermoid foci resembling abortive Hassall’s corpuscles could be observed among the neoplastic epithelial cells, but no mucin-producing cells were found in these epidermoid foci. In these areas, the neoplastic cells were diffusely positive for CK19 (c), but negative for CD5 (d). (a-b, H & E staining with original magnification of 400 ×; c-d, immunohistochemical staining with original magnification of 400 ×).