Cytokeratin 5, abbreviated as CK5, is a protein that functions as a building block within certain cells, creating the internal scaffolding (cytoskeleton) that provides structure. When a pathologist reports a “CK5 positive” result, it means a lab test has detected this protein in a tissue sample. This finding is not a diagnosis on its own but provides information to help pathologists understand the characteristics of the cells.
CK5 Expression in Healthy Tissues
To understand a CK5 positive result, it is necessary to know where this protein is normally found. CK5 is a feature of specific cell types, primarily basal cells and myoepithelial cells. These cells form foundational layers in various tissues, providing structural support and separating different tissue compartments.
A primary location for CK5-positive cells is in the deepest layer of the epidermis, the skin’s outer layer, where basal cells continuously divide to replenish the skin. Similarly, the ducts and glands within the breast are lined by myoepithelial cells that express CK5. These cells form a contractile network around the milk-producing cells.
Within the prostate gland, a layer of basal cells positive for CK5 surrounds the glandular structures, which is a feature of healthy prostate tissue. The presence of CK5 in these and other tissues, like the lining of bronchial glands and the urothelium, reflects its role in maintaining architectural integrity.
The Role of CK5 in Cancer Diagnosis
Pathologists use CK5 as a diagnostic marker to help classify different types of cancer. The primary method for detection is immunohistochemistry (IHC), where a tissue sample is treated with an antibody that binds to the CK5 protein. A chemical reaction then makes this binding visible under a microscope, staining the CK5-positive cells.
Breast Cancer
In breast cancer, CK5 staining is important for identifying “basal-like” breast cancer. These cancers are characterized by cells that share features with the basal cells of the breast ducts. A positive CK5 result is an indicator of this subtype, which is also negative for estrogen receptors, progesterone receptors, and HER2 (triple-negative).
Lung Cancer
For lung cancer, CK5 is a tool for differentiating between squamous cell carcinoma and adenocarcinoma. Squamous cell carcinoma, which arises from the flat cells that can line the airways, is CK5 positive. In contrast, adenocarcinoma, which originates from gland cells, is CK5 negative. This distinction is important as treatment strategies for these two cancers differ.
Prostate Cancer
In prostate cancer diagnosis, the absence of the protein is informative. Normal prostate glands are enveloped by a layer of basal cells that stain positive for CK5. A feature of prostate adenocarcinoma is the loss of this basal cell layer. When a pathologist finds glands that lack a CK5-positive outer layer, it is an indication of invasive cancer.
Prognostic Value of a CK5 Positive Finding
The presence of CK5 can provide information about a patient’s prognosis and guide treatment decisions. The meaning of a CK5 positive result in this context is highly dependent on the specific type of cancer.
In breast cancer, a CK5 positive finding, which helps define basal-like tumors, points to a more aggressive cancer with a higher rate of recurrence. Patients with CK5-positive tumors may have a poorer prognosis compared to those with other subtypes. This information also helps oncologists select appropriate therapies, as these tumors may be more responsive to certain types of chemotherapy.
For lung cancer, the CK5 positive result that helps confirm a diagnosis of squamous cell carcinoma directly influences the treatment path. This diagnosis has a distinct prognosis and responds differently to therapies compared to CK5-negative lung adenocarcinoma. Knowing the cancer is squamous cell type allows doctors to choose medications that are more likely to be beneficial.
The prognostic information from CK5 status is a component of personalized medicine. It allows the medical team to tailor the treatment strategy based on the specific biological characteristics of the tumor. This helps in formulating a more precise and effective plan for care.
CK5 Positive Results in Benign Conditions
A CK5 positive test result does not automatically signify cancer. Many benign conditions involve the growth of cells that are normally CK5 positive. Pathologists interpret the CK5 staining pattern in the context of the overall tissue structure to differentiate these benign growths from malignant tumors.
In the breast, a common benign condition called usual ductal hyperplasia (UDH) is characterized by a proliferation of cells within the milk ducts. These cells in UDH are CK5 positive. This contrasts with atypical ductal hyperplasia (ADH) and ductal carcinoma in situ (DCIS), which are CK5 negative. The distinct staining pattern helps a pathologist distinguish UDH from these more concerning lesions.
Similarly, in the prostate, a benign condition known as basal cell hyperplasia involves an increase in the number of CK5-positive basal cells. The preserved and emphasized CK5 staining helps confirm the benign nature of the growth. This is part of the differential diagnosis process, where the pathologist considers all possible explanations for the observed cellular changes.