Thymoma with pseudosarcomatous stroma: report of an unusual histologic variant of thymic epithelial neoplasm that may simulate carcinosarcoma.

Six cases are described of an unusual type of primary thymic epithelial neoplasm characterized by a biphasic epithelial/spindle cell morphology that closely resembled a carcinosarcoma. The patients were two women and four men 28-70 years of age. The tumors presented clinically as asymptomatic anterior mediastinal masses found incidentally on routine chest radiographs. All patients were treated by complete surgical excision. Grossly, the tumors consisted of well-circumscribed, encapsulated masses that measured 6-14 cm in greatest diameter and showed a gray-white, homogeneous, rubbery cut surface. Histologically, the lesions were composed of anastomosing islands and cords of oval to polygonal epithelial cells displaying large nuclei with occasional prominent nucleoli and rare mitotic figures, separated by areas containing a highly cellular spindle cell proliferation without nuclear atypia. Thymic remnants could be identified in the periphery of the lesions in four cases. Immunohistochemical stains showed diffuse strong positivity for keratin and focally for epithelial membrane antigen (EMA) in the epithelial cell component, and strong positivity for vimentin and focally for actin in the spindle cell stromal component. Stains for keratin, EMA, desmin, S-100 protein, and CD34 were negative in the spindle stromal cells in all cases except one, in which EMA positivity was present; CD5 stains were negative in the epithelial cells in all cases examined. Electron microscopic examination in one case showed well-formed desmosomes and tonofilaments in the epithelial elements, as well as features indicative of fibroblastic differentiation in the spindle stromal cells. Because of the unusually florid spindle cell stromal component and the focally atypical features of the epithelial cells, some of these tumors initially were misinterpreted as examples of carcinosarcoma. Clinical follow-up in five cases showed that the patients were alive and without evidence of disease over a period of 5-20 years (mean follow-up 10 years), suggesting a benign or very low grade malignant biologic behavior. The present cases appear to represent an unusual, previously undescribed morphologic variant of thymoma characterized by a prominent pseudosarcomatous stromal component. Because of the distinctive histologic appearance and indolent clinical behavior, these lesions should be distinguished from other more aggressive anterior mediastinal neoplasms displaying a biphasic morphology.