Diagnostic utility of thymic epithelial markers CD205 (DEC205) and Foxn1 in thymic epithelial neoplasms

Foxn1 and CD205 (DEC205) are novel thymic epithelial markers that are important for thymic organogenesis and the positive selection process for thymocytes, respectively. These markers were immunohistochemically applied to a total of 77 cases of thymic epithelial neoplasms comprised of 58 cases of thymomas, 17 cases of thymic carcinomas, and 2 cases of thymic neuroendocrine carcinomas. Foxn1 was diffusely expressed in nuclear staining in all cases of type B thymoma and all but 1 case of type A thymoma, whereas the expression was generally focal in thymic carcinoma (76%). The expression was identified in all cases of mixed AB thymoma, with the expression in type A component being more variable than the one in type B component. CD205 cytoplasmic expression in the form of coarse granular staining with membranous accentuation was strong and diffuse in all cases of type B thymoma (100%), and a majority of type A thymoma (89%), and focal with variable intensity in thymic carcinoma (59%). Mixed AB thymoma demonstrated diffuse expression in type B component (100%), and variable expression in type A component (94%). Neither Foxn1 nor CD205 was expressed in 2 cases of thymic neuroendocrine carcinoma. Foxn1 was focally expressed in 13% of cutaneous squamous cell carcinoma and completely negative in cutaneous basal cell carcinoma, whereas it was completely negative in squamous cell carcinoma from head and neck, esophagus and uterine cervix, and normal tissue and malignant neoplasms from all other organs other than thymus. CD205 was expressed in 4% of nonsmall cell carcinomas of lung, 27% of squamous cell carcinoma of head and neck, and 10% of squamous cell carcinoma of esophagus, but the staining pattern was different from that of thymic epithelial neoplasm and was characterized by rather homogeneous and amorphous quality without granularity or membranous reaction. CD205 was expressed in myeloid dendritic cells of various organs and tissues as well. Foxn1 is a sensitive and specific marker for thymoma and thymic carcinoma, and it appears to be superior to CD5 and CD117 for the diagnosis of thymic carcinoma. CD205 is a sensitive and specific marker for thymoma but its sensitivity to thymic carcinoma is lower than CD5 and CD117.