Spindle Cell Thymomas: An Immunohistochemical Study of 30 Cases

Spindle cell thymomas are an uncommon subset of thymomas that have been regarded as benign by some. There are no comprehensive immunohistochemical studies focusing on this histological variant of thymoma. Therefore, our knowledge on the variability of its immunohistochemical profile is limited. Thirty cases of spindle cell thymomas were reviewed and immunohistochemical studies performed using antibodies against pancytokeratin, cytokeratin 5/6 (CK5/6), CK7, CK20, thyroid transcription factor-1 (TTF-1), S-100, CD34, chromogranin, synaptophysin, epithelial membrane antigen (EMA), calretinin, desmin, smooth muscle actin (SMA), and Bcl-2. The percentage and intensity of staining were evaluated and scored. The neoplastic thymic epithelial cells were diffusely positive for CK and CK5/6 and negative for CK20, S-100, chromogranin, and desmin in all 30 cases. Eighty three percent of cases showed variable reactivity for CK7 and Bcl-2. Although 97% of cases showed focal staining for calretinin, 23% for synaptophysin, and 13% for SMA, only a single case was TTF-1 positive. An interesting staining pattern was observed for CD34 and EMA, which stained the fibrous bands in some of the cases. EMA only reacted with the epithelial cells of 1 case. Spindle cell thymomas display a strong epithelial immunohistochemical phenotype as demonstrated by their reactivity for CK and CK5/6. However, tumor cells may also show positive staining for calretinin, Bcl-2, and synaptophysin and rarely for SMA and TTF-1, which can pose a problem in the evaluation of small mediastinoscopic biopsies in separating these tumors from other more aggressive neoplasms of either epithelial or mesenchymal origin.