thymic epithelial tumors

Differential diagnosis

Rarely, primary thymic epithelial tumours may mimic mesothelioma by diffusely infiltrating the pleura to form thickened, plaque-like areas.

In the series of Attanoos et al., the thymic tumours differed in their World Health Organization type, but were classified as thymomas rather than thymic carcinomas.

The thymic epithelial component variably expresses CK5/6, calretinin, thrombomodulin and CD20, but the main distinction from mesothelioma is made on the pattern of the tumour and the lymphoid component of immature T cells, expressing CD3, CD5, terminal deoxynucleotidyl transferase (TDT, TdT) and CD99, with a high proliferation fraction.

Immunochemistry

 proteasome β subunit β5t

• Recently, a proteasome β subunit expressed exclusively in thymic cortical epithelial cells was discovered in mice and humans.
• This subunit, designated β5t, is a component of the thymoproteasome, a specialized type of proteasome implicated in thymic positive selection.
• β5t is expressed in most type B and in some type AB thymomas and is a marker useful in differentiating type B3 thymomas from thymic carcinomas when used in combination with other diagnostic markers.

Classification

Reproducibility of histopathologic classifications has some effect on outcome. Suster & Moran / S&M is the most reproducible classification. Reproducibility of invasion has no effect on the prognostic value of staging. Masaoka, Moran, and IASLC/ITMIG staging are almost perfectly reproducible. The strong correlation between Masaoka and Moran staging suggests similar prognostic strength. (25634747)

See also

 thymic tumors

References

 Reproducibility of 3 Histologic Classifications and 3 Staging Systems for Thymic Epithelial Neoplasms and Its Effect on Prognosis. Roden AC, Yi ES, Jenkins SM, Edwards KK, Donovan JL, Lewis JE, Cassivi SD, Marks RS, Garces YI, Aubry MC. Am J Surg Pathol. 2015 Jan 28. PMID: 25634747

 Expression of Proteasome Subunit β5t in Thymic Epithelial Tumors. Yamada Y, Tomaru U, Ishizu A, Kiuchi T, Marukawa K, Matsuno Y, Kasahara M. Am J Surg Pathol. 2011 Sep;35(9):1296-304. PMID: 21836487