Thymic epithelial neoplasms with sebaceous differentiation: a clinicopathological and immunohistochemical study of 8 cases

Gastric Carcinoma With Sebaceous Differentiation and SALL4 Expression

A sebaceous carcinoma is rarely seen in extracutaneous sites. We present a 75-year-old man who was admitted with epigastralgia and melena. Endoscopic examination revealed an ulcer on the posterior wall of the gastric antrum, and distal gastrectomy was performed. Histopathological examination revealed thin to thick trabeculae of polygonal cells with scattered foci of foamy cells, whereas Sudan 3 staining showed lipid vacuoles. Immunohistochemistry was positive for both p40 and SALL4. After considering these findings, we suggest sebaceous differentiation as the diagnosis. To the best of our knowledge, this is the first case of gastric carcinoma with sebaceous differentiation.

Thymoma: Histologically a heterogenous group of tumors

Over the years the nomenclature of thymomas has been debated regarding the best manner in which these tumors should be grouped. In every schema presented thus far, the main issue has been the presence or lack of lymphocytes and accordingly, the tumors have been place into a specific category. However, even though this concept applies for most of the cases, there are numerous tumors that do not necessarily fit into those categories as either the thymomas show another cellular proliferation associated with the epithelial cells or the epithelial cell themselves are arranged in a pattern that departs from the conventional features of the classic thymoma. Herein we will emphasize those features, which in some circumstances, mainly with small mediastinoscopic biopsies may pose a considerable problem in interpretation. We do consider that the most important issue is to be familiar with the different growth pattern that these tumors may show in order to avoid misdiagnosis. In addition, we consider that regardless of the growth pattern or cellular composition of the tumor, it is highly recommended that these tumors just like any other be carefully sampled and properly stage. Although we are fully aware of the different growth pattern and specific cellular details that thymoma may show, the discussion of each one of those tumors is beyond the scope of this review. Therefore, we have placed more emphasis in those, which in our judgment are more commonly encountered in the daily practice.

Potentially actionable FGFR2 high-level amplification in thymic sebaceous carcinoma

Our aim was to investigate sebaceous differentiation in thymus tumours and to identify new actionable genomic alterations. To this end we screened 35 normal and 23 hyperplastic thymuses, 127 thymomas and 41 thymic carcinomas for the presence of sebaceous differentiation as defined by morphology and expression of adipophilin and androgen receptor (AR). One primary thymic carcinoma showed morphology of sebaceous carcinomas (keratinizing and foam cells, calcifications, giant cells), a strong expression of adipophilin and AR together with squamous markers. NGS revealed high-level amplification of fibroblast growth factor receptor 2 (FGFR2). In thymuses and thymomas, no cells with sebaceous morphology were present. Occasionally, macrophages or epithelial cells showed adipophilin-positivity, however, without co-expression of AR. Thymic sebaceous carcinoma should be considered if a thymic carcinoma shows clear or foamy features. Testing for FGFR2 amplification might be warranted when searching for actionable genomic alterations in sebaceous carcinomas in the mediastinum and in other locations.