A “CK7 Positive” result is derived from immunohistochemistry (IHC), a core diagnostic technique. IHC uses specific antibodies to bind to target proteins within a tissue sample, allowing pathologists to visualize their presence. Cytokeratin 7 (CK7) is a tumor marker, and a positive result means the antibody successfully bound to a significant amount of the protein in the cells examined. This detection helps determine the identity and origin of a tumor, especially when cancer is found in a secondary site. Pathologists use the pattern of protein expression to gather clues about the tumor’s cell type of origin.
CK7’s Function as a Cellular Marker
Cytokeratins are structural proteins that form intermediate filaments inside epithelial cells, providing mechanical strength and maintaining cell shape. CK7 is a specific member of this family, primarily expressed in the epithelial cells lining glandular or ductal structures. These tissues include the linings of the lungs, breasts, ovaries, bladder, and bile ducts. Detection of CK7 in a tumor suggests the tumor originated from one of these sites, as it is consistently present in these healthy tissues.
Pathologists test for this marker to establish the lineage of a carcinoma, which arises from epithelial tissue. When a tumor has metastasized and the primary site is unknown, the CK7 status guides the diagnostic investigation. A positive CK7 result narrows the list of possible primary cancers to those derived from CK7-expressing cell types. Identifying the original tumor location is necessary for selecting the appropriate treatment plan.
Common Cancers That Express CK7
A CK7 positive result is commonly associated with a large group of adenocarcinomas, which are cancers that form in glandular tissue.
- Ovarian cancer is one of the most frequent CK7-positive malignancies, with most serous carcinomas expressing the marker.
- Lung adenocarcinoma commonly shows a strong CK7 positive staining pattern.
- Breast cancer, particularly invasive ductal carcinoma, is reliably CK7 positive, reflecting its origin in the ductal epithelium.
- Transitional cell carcinoma arises from the lining of the urinary tract, such as the bladder.
- Endometrial carcinoma originates in the lining of the uterus.
- Mesothelioma, a cancer affecting the lining of the lungs or abdomen, often stains positive for CK7, in approximately two-thirds of cases.
The consistency of CK7 expression across these different cancers is due to their shared embryological development from simple or glandular epithelial cells.
Using CK7 in Differential Diagnosis
The CK7 result is most effective when used as part of a panel alongside other markers, most notably Cytokeratin 20 (CK20). This CK7/CK20 pairing allows pathologists to differentiate between various tumor types that may look similar under the microscope, especially in cases of metastatic carcinoma of unknown primary. For example, lung or breast cancer is typically CK7-positive and CK20-negative, creating a CK7+/CK20- profile. This specific profile strongly suggests an origin in the lung, breast, ovary, or thyroid.
In contrast, colorectal adenocarcinoma is generally CK7 negative but CK20 positive, presenting a distinct CK7-/CK20+ profile. This differential staining is extremely useful in distinguishing a lung metastasis from breast cancer (CK7+/CK20-) versus a lung metastasis from colon cancer (CK7-/CK20+). Other cancers, like those of the pancreas, stomach, and mucinous ovarian tumors, can display a CK7+/CK20+ pattern, requiring additional markers to narrow the diagnosis. Classifying a tumor based on these protein expression patterns guides oncologists toward the most effective, site-specific treatment.