The second way to classify tumors is according to how fast they are growing. It would be nice if there were a simple way of doing this, like pointing a radar gun at the tumor and getting a read-out of the speed. But what usually has to happen is that a piece of the tumor is surgically removed and examined under a microscope, a process known as a biopsy . Its appearance and behavior can reveal a lot of information.
Well-Differentiated Tumors Under the microscope, some tumor cells look very much like the normal tissue they came from. If they do, they are called well-differentiated . A normal pancreas has a characteristic look, for example. A pathologist can look at a slide of tissue and see that it's a pancreas even if it is cancerous. Similarly, a follicular carcinoma of the thyroid resembles normal thyroid tissue rather well, so it is not difficult for a pathologist to know where that tumor comes from.
Undifferentiated Tumors Other tumors don't particularly look like the normal tissues they come from. They resemble the tissue of origin only slightly or not at all. They look more primitive or immature. Sometimes they don't really look like any specific tissue. These are called undifferentiated or poorly differentiated .
Looking at a piece of undifferentiated tissue under the microscope, a pathologist will probably not be able to tell where the tissue was taken from. An undifferentiated tumor of the pancreas may look the same as an undifferentiated tumor of the lung, so the pathologist has to rely on the surgeon to say what area the tissue is from.
