Desmoglein 3 and p40 immunoreactivity in neoplastic and nonneoplastic thymus: a potential adjunct to help resolve selected diagnostic and staging problems

The potential usefulness of the squamous markers p40 and desmoglein 3 (DSG-3) for the diagnosis and staging of selected thymic lesions is uncertain. We investigated their expression and distribution pattern in 66 thymomas, 12 thymic squamous carcinomas, 6 undifferentiated thymic carcinomas, 5 hyperplastic thymi, and 5 normal thymi. p40 nuclear and DSG-3 cytoplasmic/membranous immunoreactivity in greater than or equal to 10% of thymic epithelial cells was interpreted as positive, and DSG-3 distribution pattern was classified as organotypic and nonorganotypic. All nonneoplastic thymic tissues, 100% of thymic squamous carcinomas, 97% of thymomas, and 50% of undifferentiated thymic carcinomas were positive for p40. Expression of p40 in almost all thymomas and in 50% of undifferentiated carcinomas that lacked squamous features suggests that p40 is not a good marker for the diagnosis of thymic squamous carcinoma. All normal and hyperplastic thymi, 51.5% of thymomas, and 0% of thymic squamous carcinomas expressed DSG-3 in an organotypic pattern, and 13.6% of thymomas and 83% of thymic squamous carcinomas were DSG-3 positive in a nonorganotypic pattern. Findings suggest that nonorganotypic DSG-3 expression favors the diagnosis of squamous cell carcinoma over thymoma. In 26 (60.5%) of the 43 cases where neoplastic and nonneoplastic thymus were present on the same slide, the presence/absence or distribution pattern of DSG-3 immunoreactivity was different in the 2 components, suggesting that this marker can be helpful in staging thymomas with incomplete encapsulation. The presence of DSG-3-positive and DSG-3-negative thymomas raises the possibility that these tumors may originate from 2 different types of thymic epithelial cells.