Grading (also called tumour grading) describes how the cancer cells look compared to normal, healthy cells. The grade of cancer is used to help predict how the cancer will grow and to plan treatment. Doctors also use grade to predict how well a treatment will work and predict a person’s outlook (prognosis). And for some types of cancer, the grade is used to stage a cancer.
To find out the grade of a cancer, a pathologist looks at a tissue sample from the tumour under a microscope. Grading depends on several factors:
- how different the cancer cells look from normal cells (called differentiation) and other features of the tumour such as the size and shape of the cells and how the cells are arranged
- how fast the cells are growing and dividing
- whether there are areas of cell death in the tumour (called necrosis)
How cancers are graded
Some cancers have their own grading systems but most solid tumour cancers are given a grade between 1 to 3 or 4. (Solid tumour cancers, like breast or prostate cancer, form lumps.) A lower number means the cancer is a lower grade. Different parts of a tumour can have cancer cells with different grades. But the tumour is usually graded as the highest grade seen anywhere within the tumour.
- Low-grade cancers have cells that are abnormal but look a lot like normal cells. They are also arranged much like normal cells. Low-grade cancers tend to grow slowly and are less likely to spread. Cancers that are well-differentiated are low grade.
- High-grade cancers have cells that look very different from normal cells and are arranged differently. They tend to grow more quickly and are more likely to spread.
Differentiation refers to how developed the cancer cells are and how well they are organized in the tissue or organ. Cancer cells are compared to normal cells in an organ or tissue. Grade and differentiation are basically the same but grading is a standardized way of measuring differentiation. Like the grade, a tumour’s level of differentiation can change over time and different areas in a tumour can have different levels of differentiation. For most types of cancer, a grade is given based on the more undifferentiated area in a tumour.
- Well-differentiated cancer cells look and behave more like the normal cells in the tissue they started to grow in. Tumours that have well-differentiated cancer cells tend to be less aggressive. This means they tend to grow and spread slowly. Well-differentiated cancers are low grade.
- Undifferentiated or poorly differentiated cancer cells look and behave very differently from normal cells in the tissue they started to grow in. These cells look immature, undeveloped or aggressive and aren’t organized in the same pattern as normal cells. Tumours that are undifferentiated or poorly differentiated tend to be more aggressive. They tend to grow more quickly, spread more often and have a worse prognosis than tumours with well-differentiated cancer cells. Cancers that are undifferentiated or poorly differentiated are high grade.
- Moderately differentiated cancer cells look and behave somewhere between well-differentiated and undifferentiated cancer cells.
A description of the extent of cancer in the body, including the size of the tumour, whether there are cancer cells in the lymph nodes and whether the disease has spread from its original site to other parts of the body.
Stages are based on specific criteria for each type of cancer.
The process of determining the extent of cancer in the body based on exams and tests is called staging.