To our best knowledge, this is the first report of a thymic carcinoma occurring in the middle mediastinum, as demonstrated by histopathological findings with immunohistochemical features. Moreover, radiological findings demonstrating a cyst with no solid component in the middle mediastinum made preoperative diagnosis of a thymic carcinoma difficult.
Thymic carcinoma is an uncommon neoplasm and occurs in 5.5 % of all resected mediastinal tumors . Furthermore, a middle mediastinal thymic carcinoma is extremely rare. Thymic carcinoma occurs in the anterior mediastinum, and ectopic thymic carcinomas, which are carcinomas that show thymus-like differentiation, are rare . Ectopic thymic carcinoma is reported in cases of intrathyroid neoplasms , even though the first case reported was that of a middle mediastinal thymic carcinoma with histopathologic features. We think ectopic thymus tissue existed in middle mediastinum and it became a progressive neoplasm.
Thymic carcinoma is difficult to diagnose microscopically, whereas immunohistochemical features are greatly useful to make an exact diagnosis. CD5 is specific to the thymic epithelial cell, and metastatic tumor is excluded [4, 5]. To distinguish thymic squamous cell carcinoma from pulmonary squamous cell carcinoma is quite difficult. However, CD5 positive is useful for accurate diagnosis because pulmonary squamous cell carcinoma is consistently negative for CD5 by a large study of 1465 non-small cell lung cancer cases . We diagnosed thymic squamous cell carcinoma by both microscopic findings and immunohistochemical findings. Middle mediastinal thymic carcinoma diagnosed by only microscopic findings was reported . However, the immunohistochemical findings did not indicate thymic carcinoma because it was negative for CD5, which is specific to the thymic epithelial cell, and assessment for other thymic carcinoma markers (p53, c-kit, and bcl-2) was not performed. In our case, the immunohistochemical feature was coincident with thymic squamous cell carcinoma, and the possibility of other metastatic neoplasms in the mediastinum was excluded by imaging tests.
Mediastinal tumors showing cystic findings are generally benign. Malignancy has been reported in 2.4% of all cases with cystic findings detected by mediastinum CT . Focal cystic change with thymic neoplasm, occurring in 40% of resected thymomas, is usual because thymic carcinomas and thymomas contain fluid, bleeding, and necrotic tissue with degeneration . When high-grade degeneration developed, the tumor demonstrated cystic findings, such as in this case. Cystic thymomas have occasionally been reported, even though cystic thymic carcinoma is rare . The malignant component in the microscopic findings was too small to be detected by imaging tests, and it made difficult to consider malignant tumor before operation.
Thymic carcinoma can occur in the middle mediastinum. Moreover, when cystic findings are noted in the middle mediastinum, thymic carcinoma should be considered in the differential diagnosis because the operative strategy and therapy of thymic carcinoma differ from those of other benign tumors.