Pathology dictionary -
Grade is a word pathologists use to describe the difference between abnormal cells and the normal, healthy cells they have replaced.
The most common use for the word grade is to describe the difference between cancer cells and normal non-cancerous cells. However, pathologists also use the word grade to describe the changes seen in non-cancerous medical conditions.
Grade can only be determined after the tissue sample has been examined under the microscope by a pathologist.
Low and high grade cells
In most areas of the body, pathologists divide grade into two levels - low and high. Cancer cells that look, grow, and behave more like normal non-cancerous cells are called low grade. In contrast, cancer cells that look, grow, and behave less like normal non-cancerous cells are called high grade.
Why is this important? Grade is important because high grade (or more abnormal) cells are associated with worse prognosis. For this reason, grade is also used by your doctors to select an appropriate treatment.
Grade for pre-cancerous diseases
Pathologists also use grade to describe pre-cancerous diseases. In this situation, low grade diseases look more like normal, healthy cells and are less likely to turn into a cancer over time. In contrast, high grade diseases look less like normal, healthy cells and are more likely to turn into a cancer over time if not treated.
How do pathologists decide the grade?
When describing pre-cancerous or cancerous cells, pathologists look for some of the following features in order to decide the grade:
The size and shape of the abnormal cells.
The way the abnormal cells stick together.
How many of the abnormal cells are seen dividing to create new cells. The process of creating a new cell is called mitosis.
Whether or not there are any large areas of cell death called necrosis.
For some diseases, grade is divided into levels called differentiation. Differentiation is another word pathologists use to compare the abnormal cells with the cells that are normally found in that location.
Here is an example of how grade can be divided into four levels of differentiation:
Well differentiated - Well differentiated cells look very similar to the cells normally found in that location.
Moderately differentiated - Moderately differentiated cells are clearly abnormal but they still look somewhat like the cells normally found in that location.
Poorly differentiated - Poorly differentiated cells are very abnormal looking and may only share some features with the cells normally found in that location. Pathologists often order additional tests such as immunohistochemistry to figure out where the cells started.
Undifferentiated - Undifferentiated cells look nothing like normal cells and additional tests such as immunohistochemistry are almost always performed to learn more about the cells and where they started.
Grading systems for specific areas of the body
The Gleason grading system - This system is used to grade cancers of the prostate including adenocarcinoma.
The modified Scarff-Bloom-Richardson or Nottingham grading system - This system is used to grade cancers in the breast including ductal carcinoma in situ, invasive ductal carcinoma, and lobular carcinoma.
The WHO/ISUP grading system - This system is used to grade cancers of the kidney including clear cell renal cell carcinoma.