Tumour eosinophilia combined with an immunohistochemistry panel is useful in the differentiation of type B3 thymoma from thymic carcinoma.

It is sometimes difficult to differentiate between type B3 thymoma from thymic carcinoma histologically. Given the rarity of these tumours, studies have been limited. A series of 66 thymic neoplasms were reviewed and classified according to the World Health Organization (WHO) scheme. We performed a tissue microarray analysis of surgically resected thymic tumour specimens including 12 thymic carcinomas, 17 type B3 thymomas and 37 thymomas of other types. Percentage and staining intensity of immunohistochemical markers were recorded. Tumour eosinophilia was recorded positive if at least one eosinophilic cell identified. Positive staining of the following markers significantly differentiated type B3 thymoma from thymic carcinoma: cytokeratin 5/6 (15 vs. 3), Mesothelin (0 vs. 5), cytoplasmic androgen receptor (10 vs. 0), CD57 (9 vs. 0), CD5 (0 vs. 7), TdT (lymphocytic) (14 vs. 1), CD1a (lymphocytic) (14 vs. 2), CD117 (1 vs. 9), MOC31 (2 vs. 6), p21 (2 vs. 8), cytoplasmic Survivin (0 vs. 4), and tumour eosinophilia (1 vs. 11). Combining two or three markers was able to differentiate these two tumours with area under the curve percentage of at least 92%. Tumour eosinophilia combined with a panel of immunohistochemistry could differentiate type B3 thymoma from thymic carcinoma.