Grading is a way of classifying cancer cells based on their appearance and behaviour when viewed under a microscope. The doctor needs to know how different the cancer cells look from normal cells, how frequently they are dividing (mitoticmitoticThe process by which a cell divides to form 2 new cells. Each new cell receives a complete set of chromosomes from the original cell. activity) and how likely they are to spread to new locations.
Differentiation refers to how cancer cells look and function compared to normal cells. A tumour's level of differentiation can vary with time and can be described in degrees.
- Well-differentiated cancer cells look and behave like the normal cells in the tissue they started to grow in. Tumours that contain well-differentiated cancer cells tend to be slow growing and less aggressive.
- Undifferentiated or poorly differentiated cancer cells look and behave quite differently from normal cells in the tissue they started to grow in. They look immature or undeveloped and often do not resemble the tissue of origin at all. Tumours that contain undifferentiated or poorly differentiated cancer cells are more aggressive. They tend to grow quicker, spread more often and have a worse prognosis than tumours with well-differentiated cancer cells.
- Moderately differentiated cancer cells look and behave somewhere between well-differentiated and undifferentiated cancer cells. Most types of cancer have moderately differentiated cancer cells.
Some tumours are described as low grade or high grade. Their grade is based on their degree of differentiation and their growth rate.
- Low-grade cancer cells are usually well differentiated and the tumours are slower growing.
- High-grade cancer cells are usually poorly differentiated or undifferentiated, and the tumours are faster growing and spread earlier.
Sometimes tumour grade is described with a number between 1 and 4. The number refers to the degree of differentiation:
- The lower the number, the lower the grade.
- The higher the number, the higher the grade.
- A large tumour may contain cells of different grades.
grade cannot be assessed
Different grading systems may be used for different types of cancer. Some types of cancer do not have specific grading criteria. For most types of cancer, grading plays an important part in planning treatment and can also be used to help estimate the future outcome (prognosis). Generally, a lower grade represents a more favourable prognosis. However, grading may not be particularly useful for some types of cancer.