Ki67 is a special stain that gives a sense of how aggressive a tumor is. The pathologist takes the biopsy or surgical specimen, prepares it, puts it on to a glass slide, stains it for this protein, and look at it under the microscope. It's a protein in cells that is involved in cell replication, so if many cells are expressing it, the tumor is growing quickly; if very few cells express it, then it is growing slowly. The pathologist needs to count about a thousand cells and determine the percentage of cells that are Ki67 positive. Therefore, the number that comes back should be a percentage from 0 - 100%.
We use this number in neuroendocrine to get a feeling of how aggressive the tumor is. In general, if the Ki67 is between 0-2%, then we call it grade 1 or low grade. If it is between 2-20%, we call it grade 2 or intermediate grade. If it is > 20%, then it is grade 3 or high grade. The good, the bad, and the ugly.
However, the devil is in the details. First of all, the stain needs to be ACTUALLY DONE. It is not routine, so many people don't have it on their specimens. Second, what type of specimen was it performed on? Tiny little samples may not have enough cells to make it meaningful. Third, where was the sample taken from?
This whole topic came up because most people "generally" consider Ki67 to be consistent within the same person from tumor to tumor. When I started doing neuroendocrine, I found this not to be the case in my patients. Therefore, we stained many tumors from several patients and found there is "heterogeneity" in the patients. In other words, one patient could have Ki67 from 1% in the primary tumor, but 30% in a liver metastasis. Well, that certainly made things more complicated.
That's why I like to re-review pathology specimens and why I so strongly advocate for seeing a NET specialist. It's complicated - I know.
Eric Liu, M.D.