Utility of immunohistochemistry in distinguishing primary adenocarcinomas from metastatic breast carcinomas in the gastrointestinal tract.

Breast carcinoma often metastasizes to the gastrointestinal tract, especially the stomach, where it is frequently difficult to distinguish from a primary gastric carcinoma. To evaluate the utility of immunohistochemical stains in differentiating primary gastric carcinomas from metastatic breast carcinomas. Mucosal biopsy specimens from 47 adenocarcinomas involving the gastrointestinal tract (28 primary gastric carcinomas and 19 metastatic breast carcinomas) and 16 control cases of primary breast carcinomas without metastasis were immunohistochemically stained for estrogen receptor protein (ER), progesterone receptor protein (PR), gross cystic disease fluid protein (GCDFP), human epidermal growth factor receptor 2 protein, cytokeratin (CK) 5/6, CK/7, CK/20, a panel of mucin glycoprotein antigens (MUC2, MUC3, MUC5AC, and MUC6), monoclonal antibody DAS-1, and caudal-type homeobox transcription factor CDX2 and compared between primary and metastatic adenocarcinomas. Highly significant proportions of metastatic breast carcinomas were positive for ER (72%), PR (33%), GCDFP (78%), and CK5/6 (61%) compared with primary gastric carcinomas (ER, 0%; PR, 0%; GCDFP, 0%; and CK5/6, 14%) (P < .001, P = .002, P < .001, and P = .004, respectively). Of these immunostains, ER, PR, and GCDFP were 100% specific. Primary breast tumors and their metastases showed a similar phenotypic profile. In contrast, primary gastric carcinomas showed significantly higher proportions of cases that stained with CK20 (50%), MUC2 (54%), MUC5AC (71%), MUC6 (39%), DAS-1 (43%), and CDX2 (67%) compared with metastatic breast carcinomas (CK20, 0%; MUC2, 24%; MUC5AC, 6%; MUC6, 0%; DAS-1, 0%; and CDX2, 0%) (P = .001, P = .01, P < .001, P = .02, P = .009, and P < .001, respectively). No significant differences were observed with regard to any of the other immunostains (human epidermal growth factor receptor 2 protein, CK7, and MUC3) between the patient groups. Estrogen receptor protein, PR, GCDFP, CK5/6, CK20, MUC5AC, MUC6, DAS-1, and CDX2 are helpful in distinguishing primary gastric carcinomas from metastatic breast carcinomas. Of these, ER, PR, and GCDFP are highly specific for metastatic breast carcinomas, whereas CK20, DAS-1, MUC2, MUC5AC, MUC6, and CDX2 are highly specific for primary gastric carcinomas.