Patterns of distribution of cytokeratins 20 and 7 in special types of invasive breast carcinoma: a study of 123 cases.

Metastatic adenocarcinomas of unknown primary site are a common clinical problem. Invasive ductal carcinomas of the breast and some special types of invasive breast carcinoma are common sources of metastases. Immunohistochemical algorithms, such as a combination of cytokeratins 20 and 7, can be helpful in this situation. Detailed phenotyping of the different types and subtypes of primary invasive carcinomas and their metastases is an essential prerequisite for a successful search for an unknown primary tumor. A series of 123 primary invasive breast adenocarcinomas of special type and of 27 lymph node metastases was analyzed. Sections of selected blocks were stained with two monoclonal cytokeratin antibodies (CK20 and CK7) and evaluated as negative (no staining), focally positive or diffusely positive. Of the 123 carcinomas, 113 (92%) proved to be CK20 negative. Three of 82 (4%) invasive lobular carcinomas, three of 11 (27%) mucinous carcinomas, one of 10 (10%) tubular carcinomas, and one invasive papillary carcinoma stained diffusely with CK20. Additionally, a tubulolobular carcinoma and a medullary carcinoma showed focal CK20 positivity. One hundred twenty (98%) of the 123 tumors were CK7 positive, five of them only focally. One of the four solid invasive lobular carcinomas, one medually carcinoma, and one invasive papillary carcinoma were completely negative for CK7. Only two cases, one mucinous and one invasive papillary carcinoma, exhibited the CK20(+)/CK7(-) ("colorectal") pattern. One of the lymph node metastases was CK20(+); another was CK7(-). Like their ductal counterparts, invasive breast carcinomas of special type are usually CK20(-)/CK7(+); they generally retain this phenotype in their metastases. However, there are CK20-positive special-type breast carcinomas that can be confused with gastrointestinal or pancreaticobiliary carcinoma in metastases, especially if they are mucinous or invasive lobular.