Thymic basaloid carcinoma: a clinicopathologic study of 12 cases, with a general discussion of basaloid carcinoma and its relationship with adenoid cystic carcinoma.

Thymic carcinoma (primary carcinoma of the thymic epithelium; type C thymoma) is a rare malignancy. It usually presents in middle-aged to elderly patients and can exhibit a wide variety of morphologic appearances. Thymic basaloid carcinoma (thymic BC) is a particularly rare subtype, with less than 20 cases published in the English literature, mostly in the form of individual case reports. In this study, we present the clinicopathologic and immunohistochemical features of 12 new cases of thymic BC. There were 10 (83%) men and 2 (17%) women. Ages at the time of initial diagnosis ranged from 34 to 77 years (mean 55 y). The 2 most common manners of presentation were dyspnea on exertion (3 patients) and as an incidental finding on radiographic imaging (2 patients). Tumors ranged in size from 4.4 to 17 cm (mean 10.1 cm). One of 12 cases (8.3%) was associated with a multilocular thymic cyst. Immunohistochemistry was performed in 8 cases. Pan-cytokeratin was positive in all cases. CD117 (c-kit) was positive in 6 of 8 cases (75%), p63 was positive in 7 of 8 cases (88%), p53 was positive in 7 of 8 cases (88%), ranging from <10% to 90%, CD5 was focally positive in 3 of 8 cases (38%), collagen type IV was positive in 4 of 8 cases (50%), and proliferative index, as estimated by Ki67, ranged from <1% to approximately 15%. In 1 of 2 cases with sarcomatoid differentiation, Ki67 was greater than 80% in the sarcomatoid area. Cases were negative for thyroid transcription factor-1 (0 of 8), S-100 (0 of 7), and synaptophysin (0 of 7). Long-term data was available in 8 patients with an average follow-up of 30 months. Five patients died of their disease at an average of 34 months from the time of diagnosis. Of the remaining 3 patients, 1 had a stable recurrence and died at 4 years from unrelated causes, and 2 were alive without the evidence of disease at 12 and 7 months, respectively. Thymic BC, although previously regarded as a low-grade neoplasm, has shown that it is capable of aggressive behavior and significant mortality. In this paper, we review the pertinent literature and discuss the possible relationship of thymic BC with thymic adenoid cystic carcinoma, as well as BCs and adenoid cystic carcinomas at other sites.