Original ArticlesSpindle Cell and Mixed Spindle/Lymphocytic Thymomas An Integrated Clinicopathologic and Immunohistochemical Study of 81 CasesPan, Chin-Chen M.D.; Chen, Winby York-Kwan M.D.; Chiang, Hung M.D.Author Information From the Department of Pathology, National Yang-Ming University and Veterans General Hospital-Taipei, Taiwan. Address correspondence and reprint requests to Chin-Chen Pan, MD, Department of Pathology, Veterans General Hospital-Taipei, No.201, Sec. 2, Shih-Pai Rd., Taipei, 11217, Taiwan; e-mail: [email protected] The American Journal of Surgical Pathology: January 2001 - Volume 25 - Issue 1 - p 111-120 Buy Abstract Forty-three cases of spindle cell thymoma (medullary, WHO type A) and 38 cases of mixed spindle/lymphocytic thymoma (WHO type AB) were studied for their clinicopathologic and immunohistochemical characteristics. Three histologic patterns of spindle cell thymoma were observed: short-spindled (57%), long-spindled (31%), and micronodular (12%). The short-spindled variant was composed of oval to short spindle cells commonly arranged in a hemangiopericytic or microcystic pattern. The long-spindled variant chiefly consisted of fibroblast-like epithelial cells mimicking fibrohistiocytic neoplasms. The micronodular variant was characterized by small nests of short spindle cells dispersed among a lymphoid stroma with frequent germinal centers. All kinds of spindle cell could be admixed with lymphocyte-rich “cortex”-like areas to constitute mixed spindle/lymphocytic thymomas. Immunohistochemically, the epithelial cells in up to 70% of the short-spindled and long-spindled variants of spindle cell thymoma and 90% of mixed spindle/lymphocytic thymomas were positive for CD20, whereas the epithelial cells in all micronodular spindle cell thymomas were negative. All of the spindle cell thymomas and most of the mixed spindle/lymphocytic thymomas in this study were found in stages I and II. Follow up of the patients did not disclose relapse or mortality directly resulting from the tumors. However, the prognosis of stage I and II spindle cell and mixed spindle/lymphocytic thymomas did not significantly differ from those of stage I and II thymomas of other types by a stage-matched survival analysis. Our data showed that spindle cell and mixed spindle/lymphocytic thymomas are distinctive in histologic pattern and immunohistochemical profile. When interpreted within the context of staging, spindle cell and mixed spindle/lymphocytic thymomas presenting in stages I and II most likely behave in an indolent fashion. © 2001 Lippincott Williams & Wilkins, Inc.