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 sire of origin. Fifty-one cases in which CK 7/20 immunocytochemistry was performed as part of rite 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 carcino mas, the least informative. The second most common immunophenotype was CK 7 -/20+, which is associated with colorectal origin, and as such was very use ful when obtained. The CK immunoprofile was more useful in the setting of a prior carcinoma, b eing a major diagnostic determinant in 13 cases (55%) from group 1 (those w ith 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 ex-peered immunoprofiles (e. g,, CK 7+/CK 20- carcinomas, including lung, breast, ovary, endometrium, an d others, vs. CK 7-/CK 20+ carcinomas, primarily colorectal). When similar immunoprofiles are obtained, their usefulness is greater if they are immuno profiles other than the most common 7+/20- pattern. Similarly in newly diag nosed carcinomas, the CK immunoprofile either helps to narrow the different ial diagnosis or points to a specific diagnosis. Diagn. Cytopathol. 1999;20 :63-66. (C) 1999 Wiley-Liss, Inc.