Since I am not medically certified to explain this topic, I used credentialed resources to gather and summarize this complex topic.  The difference between grading and staging in a breast cancer diagnosis can be confusing since the common way to refer to cancer progression is by staging.  Grading is completely different and warrants understanding to understand the aggressiveness of the cancer.

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Staging is a way of describing or classifying a cancer based on the extent of cancer in the body. The stage is often based on the size of the tumor, whether the cancer has spread (metastasized) from where it started to other parts of the body and where it has spread. Stages are based on specific factors for each type of cancer. (TNM staging system – T = Tumor; N = node involvement and M = metastastic spread)

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Grading is a way of classifying cancer cells. The pathologist gives the cancer a grade based on how different they look from normal cells (differentiation), how quickly they are growing and dividing, and how likely they are to spread. (How aggressive is the cancer?)

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Some tumors 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 tumors are slower growing.

• High-grade cancer cells are usually poorly differentiated or undifferentiated, and the tumors are faster growing and spread earlier.

• Sometimes tumor 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 tumor may contain cells of different grades.

• Tumor grades

GX – – grade cannot be assessed
G1 – well differentiated –  low grade
G2 – moderately differentiated – intermediate grade
G3 – poorly differentiated – high grade
G4 – undifferentiated – high grade

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Physicians use the grade of the cancer to figure out how slowly or quickly the cancer may be growing.

A really good visual explanation of breast cancer grade and staging can be found at:

http://www.nationalbreastcancer.org/breast-cancer-stages

Physicians use the stage and grade of the cancer, as well as other factors, to help plan treatment, estimate how the cancer might respond to treatment and give a prognosis (the expected outcome or course of a disease).