Thymomas with prominent signet ring cell-like features: a clinicopathologic and immunohistochemical study of 10 cases.

Ten cases of an unusual growth pattern of thymomas are presented. The patients were 8 men and 2 women between the ages of 43 and 62 years. Clinically, 6 patients presented with symptoms of chest pain, cough, and shortness of breath, whereas 4 patients were asymptomatic. Surgical resection of the mediastinal mass was performed in all patients obtaining tumors that varied in size from 4 to 11 cm in greatest dimension. Grossly, the tumors were described as round and well defined, which, at cut surface, showed a firm consistency with a slight lobulated appearance. Areas of necrosis and/or hemorrhage were not present. Histologically, all the tumors showed similar growth pattern characterized by the presence of cells arranged in small cords dissecting fibroconnective tissue and, in some areas, resembling "signet ring" cells. Mitotic activity was not present. Using the Masaoka staging system, 6 cases were in stage I, and 4 cases were in stage II. Histochemical stains for periodic-acid Schiff stain with diastase and mucicarmine were negative for intracytoplasmic mucin. Immunohistochemical studies for CAM5.2 and cytokeratin 5/6 showed a strong positive reaction, whereas thyroid transcription factor 1, epithelial membrane antigen, calretinin, α-fetoprotein, and CD31 were all negative. Follow-up information obtained for 7 patients between 1 and 12 years showed that all these patients are alive and well. The current cases highlight an unusual histologic growth pattern of thymomas that likely can be confused with other tumors of the anterior mediastinum that may require different treatment approaches and carry a different prognosis.