Distinction of thymic carcinoma and type B3 thymoma using ancillary biomarkers

Objective. Thymic carcinomas (TC) are rare and understudied tumors. Pitfalls exist with TC diagnosis, and biomarkers are needed to support the pathologist. Here, we tested a series of biomarkers to differentiate TC from type B3 thymoma. Methods. A consecutive series of 48 patients, 26 with TC and 22 with type B3 thymoma entered the study. Immunohistochemical expression of CD5, CD117, BAP1, MTAP, Ki-67 was evaluated. CDKN2A status was assessed by FISH. Results. CD5 and CD117 were expressed in TC only (n = 19 and n = 20 respectively). Five TC did not show CD5 or CD117 expression. BAP1 expression was lost in 3 TC, while MTAP staining was absent in 3 TC and 1 type B3 thymoma. CDKN2A deletion was observed in 4 TC and 1 type B3 thymoma. CD5 and CD117 showed a perfect specificity for TC and a good sensitivity, especially when combined (0.81). The addition of the other markers improved the sensitivity (0.85) with a slight decrease in specificity (0.95). Indeed, one type B3 thymoma harboured CDKN2A deletion with MTAP loss of expression. Conclusions. CD5 and CD117 are the best markers for TC. While the addition of other markers (i.e., BAP1 loss, MTAP loss and CDKN2A deletion) might be useful in cases negative for CD5 and CD117, rare cases of type B3 thymoma might harbor these alterations.