The grade of a cancer is usually only determined after the tumour has been removed and examined by a pathologist. The grade of cancer relates to how healthy or unhealthy the cells look under a microscope. In other words, a pathologist will determine the grade of cancer by comparing the amount of the healthy-looking tissue to the amount of cancerous tissue. If most of the tumour cells look like normal tissue, then the cancer is “well-differentiated” or “low-grade.” However, if the tumour cells look very different from normal tissue, then the cancer is considered “poorly-differentiated”, “undifferentiated”, or “high-grade.” The grade of the cancer may help to predict how quickly the cancer may spread.
The focus of this section is on well-differentiated thyroid cancers, which are, by definition, considered to be of a lower grade. However, some “well-differentiated” thyroid cancers may have portions that are “poorly differentiated.” Adjustments to postoperative treatment may be made if this is found on final pathology. Overall, this is an uncommon scenario.