Utility of cytokeratin 7 and 20 subset analysis as an aid in the identification of primary site of origin of malignancy in cytologic specimens.

This study was undertaken to assess the utility of combined cytokeratin (CK) 7/20 immunoprofile determination in malignant cytologic cell blocks as an aid to the identification of tumor primary site of origin. Fifty-one cases in which CK 7/20 immunocytochemistry was performed as part of the initial workup were retrieved. Their contribution to the final cytologic diagnosis of tumor primary site of origin was analyzed. CK reactivity patterns were 7+/20- (n = 34), 7-/20+ (n = 9), 7-/20- (n = 7), and 7+/20+ (n = 1). The CK 7+/CK 20- immunophenotype was the most common one obtained, and due to its wide expression in a number of common carcinomas, the least informative. The second most common immunophenotype was CK 7-/20+, which is associated with colorectal origin, and as such was very useful when obtained. The CK immunoprofile was more useful in the setting of a prior carcinoma, being a major diagnostic determinant in 13 cases (55%) from group 1 (those with a prior history of malignancy), compared to 8 cases (29%) from group 2 (those with no prior history of malignancy). In the setting of prior carcinoma, the CK immunoprofile is most useful when carcinomas under consideration have different expected immunoprofiles (e.g., CK 7+/CK20- carcinomas, including lung, breast, ovary, endometrium, and others, vs. CK 7-/CK 20+ carcinomas, primarily colorectal). When similar immunoprofiles are obtained, their usefulness is greater if they are immunoprofiles other than the most common 7+/20- pattern. Similarly, in newly diagnosed carcinomas, the CK immunoprofile either helps to narrow the differential diagnosis or points to a specific diagnosis.