Expression of alpha-smooth-muscle actin antigen in thymoma and thymic cancer

Using specimens from 95 patients with thymoma and 6 with thymic cancer, we studied the expression of an alpha-smooth-muscle actin antigen immunohistochemically. It was mainly identified in the cytoplasm of small spindle cells within stroma and in the cytoplasm of tumor cells excepting vessels. In thymomas, the stromal expression is related to poor survival, while the cytoplasmic expression within tumor cells correlate with good outcome. Our data indicate that significance of the smooth muscle differentiation may be different between stromal and tumor cells. This antigen may be a useful indicator of survival in thymic neoplasms, especially in thymoma.