Quick Read: Cancer Staging--The Basics of TNM

Updated: May 23

Oncologists will say that your cancer is Stage 0, I, II, III or IV. But how do they arrive at that designation?

Behind the scenes, they often use a more detailed staging system to arrive at the overall stage. Upon reading your pathology report, you may see something like "T2N1M0." Seeing this TNM scoring system is the metaphorical peek behind the curtain.

Before we get into the details, though, I need to put on my old science teacher hat and explain the lymphatic system and how it relates to cancer spread. I’ll keep it simple.

The Lymphatic System

Our lymphatic system is part of our immune system. Tiny lymphatic vessels run alongside our blood vessels. Periodically, lymphatic vessels are interrupted by lymph nodes, where certain immune functions take place.

Potentially damaging foreign agents (that could be chemicals, molecules, viruses, bacteria, cancer cells, etc.) are identified in the lymphatic vessels for closer inspection and it’s at the nodes where they are often isolated and killed.

Lymphatic vessels run alongside our arteries and veins.

I’ll draw an analogy.

Sometimes a truckload of fruit might be transporting harmful fruit flies across state lines and they have to be found before they infest a faraway crop.

You can think of a highway (lymphatic vessels) that's periodically dotted by inspection stations (lymph nodes). Big rigs, which may or may not be carrying infested fruit (cancer cells), have to stop at an inspection station. The inspectors at the station (white blood cells inside the lymph nodes) confiscate and discard any illegal fruit. When lots of illegal fruit is being transported simultaneously, many trucks get backed up at the inspection station (swelling) and it signifies that there is a very infested crop (cancer) somewhere out there.

There are a few key things to remember here:

1) As cancer progresses, cancer cells leave the original tumor and are often pulled into a lymph node for closer inspection. This is one of the first stages of cancer "metastasis" (or spread).

2) Lymph nodes collect foreign materials, including these cancer cells, in an attempt to isolate them, kill them and alert the entire immune system to go find and kill the source of the cells.

3) If there are many cancer cells flowing though the lymphatic system, the lymph nodes closest to the original (often called the "primary") tumor swell up.

4) This swelling--the collection of cancer cells--can sometimes be felt by your doctor. (For example, if you were to have throat cancer, cancer cells might collect in nearby lymph nodes. In this case, it's the same lymph nodes that a doctor feels under your jaw when you have a cold. Primary tumors in other locations often require other methods of lymph node examination).

Lymph Nodes. We have 600 to 700 of these babies working for us every day.

TNM Staging

Now we can define the T, N and M.

T = Tumor

N = Node

M = Metastasis

The oncologist is actually staging each one of these aspects of your cancer with similar numbers used in the overall stage. Each letter receives a number next to it.

The T number generally refers to how large the tumor is and whether or not it has invaded nearby tissues.

The N number refers to how many lymph nodes have cancer cells in them and how large those lymph nodes have grown.

The M number refers to how much metastasis, the spread of cancer to other parts of the body, has occurred.

As the numbers increase, it indicates that the cancer is maturing.

So let's say that original example--T2N1M0--was for breast cancer.

If you look at the NCCN guidelines, T2 specifies that the tumor is between 2 and 5 cm at its widest diameter. (For reference, that's between about 3/4 of an inch and 2 inches).

The N1 indicates that the cancer has spread to between 1 and 3 "axillary" (or simply, underarm) lymph nodes. Or it may be the case that mammary lymph nodes had signs of cancer (which would have been discovered upon analyzing the biopsy).

Finally, the M0 means that the original tumor has not metastasized.

A Few More Tidbits

1) These T, N and M stages are almost always used together to give you a simplified, less detailed stage.

2) There are other types of staging systems. TNM is the most commonly used, but it generally isn’t used for blood cancers or brain and spinal cord tumors.

3) There are other things that you might see on a pathology report. If you see an “X” on your pathology report next to T, N or M, it indicates that its stage is unknown.

4) You should also know that the tumor grade is different than the stage. The grade tells us how slowly or quickly the cancer is expected to grow. The grade, in addition to TNM staging, helps determine the overall stage.

5) Some cancers--and this is true for breast cancer--use factors in addition to grade and TNM to determine stage. For breast cancer, it's the ER, PR and HER-2 status that is relevant here.


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