A 66-year-old woman, who had complained of left ptosis and been diagnosed as having
myasthenia gravis (ocular type) 9 years previously, was admitted to our hospital for further examination of an abnormal mass shadow on chest
X-ray film. Chest computerized tomography (CT) revealed a cystic
tumor in the anterior mediastinum and
thoracotomy was performed to make a definitive diagnosis. The
tumor could be removed completely and was diagnosed histologically as a
thymic cyst with
thymoma. The
cyst was 32 x 30 x 28 mm in size and contained about 10 ml of brown turbid fluid. The levels of CEA (61.3 ng/ml), SLX (24,000 U/ml) and CA125 (1300 U/ml) were markedly increased in the cystic fluid, while those in serum were not elevated. Immunohistochemical staining demonstrated the epithelial cells lining the
cyst to contain SLX. The findings suggest that SLX was produced in the epithelial cells of the
cyst. However, neither CEA nor CA125 was positive in the epithelial cells. From our review of reported cases of
thymic cyst showing elevated levels of
tumor markers in the cystic fluid, the main mechanism of marked elevation of
tumor markers seems to be condensation in the cystic fluid rather than increased production in epithelial cells lining the
cyst. Cases of
thymic cyst with
thymoma are extremely rare, but our case was concluded to be incidentally associated with non-invasive
thymoma.