IMMUNOHISTOCHEMICAL STAINING FOR BCL-2 AND MCL-1 IN INTRATHYROIDAL EPITHELIAL THYMOMA (ITET) CARCINOMA SHOWING THYMUS-LIKE DIFFERENTIATION (CASTLE) AND CERVICAL THYMIC CARCINOMA/

Intrathyroidal epithelial thymoma (ITET)/ carcinoma showing thymus-lik e differentiation (CASTLE), a rare thyroid neoplasm, was recently show n to be immunoreactive for CD5, providing immunophenotypic evidence of previously postulated thymic differentiation. To assess whether ectop ic malignant neoplasms with thymic differentiation display other marke rs associated with thymic carcinoma, we studied five cases of ITET/CAS TLE, two cases of cervical thymic carcinoma, and one case of cervical thymoma for bcl-2 and mcl-1 immunoreactivity, Both of these antiapopto sis proto-oncogenes have been reported to be expressed by the majority of true thymic carcinomas but only a minority of thymomas, All of the five cases of ITET/CASTLE, both CDS-positive cervical thymic carcinom as, and one CD5-negative cervical thymoma were immunoreactive for bcl- 2, as were 10 (91%) of 11 thymic carcinomas arising in the thymus, in contrast to 6 (25%) of 24 benign and invasive thymomas arising in the thymus. Similarly, all of the five cases of ITET/CASTLE, both cervical thymic carcinomas, but not the cervical thymoma, were immunoreactive for mcl-1, as were 9 (90%) of 10 thymic carcinomas, in contrast to 6 ( 33%) of 18 benign and invasive thymomas, We conclude that dual immunor eactivity for bcl-2 and mcl-1 is a feature of malignant neoplasms with thymic differentiation in general, both within the thymus and at ecto pic sites.