Cancer Staging:

The next step is to stage the disease. The classical way of staging the disease depends on T (tumor size) N (number of lymph nodes involved with cancer) and M (presence of distant disease). This TNM classification provides the basis of the staging system. The bigger the T, the greater the number of involved lymph nodes, the presence of metastatic disease increases the stage from the lowest (and best) of 1 to the highest (worst) of 4.

Lymph nodes are like service stations for immunity. E.g. Lymph nodes (or glands) in your neck swell up when you have a sore throat. When there is cancer in a particular organ, the lymph glands that normally service that area (armpit for breast, e.g.) are examined to see if the cancer cells have travelled from the primary tumor site. If that barrier has not been breached, that bodes well that the cancer is contained in the primary tumor area.

Cancer is always known by the site of primary origin: the sites of spread are called metastatic disease. If breast cancer goes to the bone, it is not bone cancer, but breast cancer metastatic to bones. The cancer cell in the bone still responds to breast cancer treatments.

These three components are put together in a combination to determine Stage:

e.g. T1 N0 M0 will be stage 1, and  Tx (any T) N x (any N) M1 (distant or disease that has spread, or metastasized) is Stage 4.  Stage 2 and 3 are various combinations in between.

The purpose of staging is that the correct treatment be chosen for that stage.