ETD

• bap1
• cdkn2a
• mtap
• thymic carcinoma
• type b3 thymoma

Thymic carcinoma is characterized by some overlapping features with type B3 thymoma. Due to the different outcome, several markers were assessed to better characterize thymic carcinoma.
BAP1, mTAP, Ki67, CD5 and CD117 immunohistochemical analysis and CDKN2A fluorescent in situ hybridization were performed on tissue from 27 thymic carcinoma and 23 type B3 thymoma patients at the University Hospital of Pisa. Microsatellite instability testing and c-KIT mutation analysis were also conducted on some thymic carcinomas.
Four of the thymic carcinomas showed loss of BAP1. Moreover, 3 CDKN2A homozygous deletions were highlighted. Interestingly, mTAP, a protein encoded by a gene in close proximity to CDKN2A on 9p21.3, was, except in 3 cases, always positive, showing, in two subjects, a concordant expression with CDKN2A deletion. Moreover, Ki67 revealed a high response variability between 10 and 95%. CD117 and CD5 exhibited a high co-expression: indeed, CD117 was positive in 77,77% and CD5 in 70,37% of thymic carcinomas, with a 40-99% and 20-90% protein expression range, respectively, in addition to, in any case, microsatellite stability and c-KIT wild-type. As regards type B3 thymomas, a significant difference compared to carcinomas in CD5 (13%) and CD117 (0%) expression was revealed.
CD5 and CD117 assessment continue being unavoidable in thymic carcinoma – type B3 thymoma differential diagnosis; BAP1 and mTAP appear useful just in selected cases. These data encourage the investigation on the limen between these two neoplastic entities.