Physicians use the staging process to assess the size and location of the cancer. Knowing the cancer stage is an important factor in selecting treatment options.

Cancer staging is based on:

• Size of the tumor
• Whether the cancer is invasive or non-invasive
• Whether lymph nodes are involved
• Whether the cancer has spread

The purpose of the staging system is to help organize the different factors and some of the personality features of the cancer into categories, to:

• Understand the most likely outcome of the disease
• Guide treatment decisions

Stages of Breast Cancer (National Cancer Institute)