If you or someone you know has been diagnosed with cancer, chances are you’ve been told that they’re at a certain stage of the disease.

"The main purpose for staging is to evaluate the extent of cancer spread," and determine a patient's prognosis, says gynaecologic oncologist Dr Ali Mahdavi. In other words, it gives the doctor information on how the disease is likely to progress over time, what symptoms to expect, what the survival rate is, and what some of the potential complications could be. Generally speaking, the earlier the stage of the cancer, the better the prognosis.

Staging also dictates treatment options. For instance, if an early-stage cancer is located in one particular area, it may be easier to remove surgically or treat with radiation as opposed to a late-stage cancer that has spread to other areas of the body and may require chemotherapy.

The most common cancer staging system: TNM

The most widely used cancer staging system is called the TNM classification system. This system assigns letters and numbers to the cancer to describe the tumour (T), the lymph nodes (N), and how much the cancer has metastasised (M). These individual evaluations help determine the overall stage of the cancer.


The T category in TNM provides information about the tumour, like its size, how much it has grown into the organ it originated from, and whether it has spread to other tissues.

  • TX means the tumour can't be measured.
  • T0 means the tumour can't be found.
  • Tis means the cancer is growing only in the most superficial layer of tissue (this is usually true with Stage 0 or in situ cancer).
  • T1-T4 describes the size of the tumour and the extent to which it has spread to nearby tissues. The higher the number after the T, the larger the tumour.

Lymph nodes

The N category describes whether the cancer has spread to the lymph nodes.

  • NX means the nearby lymph nodes can't be evaluated.
  • N0 means nearby lymph nodes don't contain cancer.
  • N1-N3 describes the size, location, and/or the number of nearby lymph nodes affected by the cancer.


Metastasis shows whether the cancer has spread to other parts of the body.

  • MX means that the metastasis can't be measured.
  • M0 means that the cancer hasn’t spread widely.
  • M1 means that the cancer has spread to distant organs or tissues.

Cancer stages 0 to IV

Once T, N, and M are determined, a doctor will assign the cancer a stage from zero to four. Staging differs a bit from cancer to cancer-sometimes stages are subdivided into A and B categories, for instance-but here's a ballpark overview of how each stage is diagnosed.

Stage 0 cancer

Truth be told, whether or not stage 0 is the first and earliest stage of cancer or a precancerous stage is up for debate. (And not all cancers have a stage 0.) But once you get past the semantic argument, this is the gist:

"If we think of precancerous as a spectrum, from slightly abnormal cells on one end to very abnormal precancerous cells on the other end, stage 0 is the severest form of precancer on the extreme end of that spectrum," says gynaecologic oncologist Dr June Hou.

For most cancers, stage 0 is called carcinoma in situ (CIS). "Stage 0 typically means that there are cancer cells that haven't penetrated through the lining or the initial wall," says oncologist Dr David Oubre. "For instance, if it's breast cancer, you can get something called DCIS: ductal carcinoma in situ. In that situation, the cancer hasn't spread from the milk duct into the regular breast tissue."

But the in situ phase is still dangerous. "In time, they'll eventually become invasive. So there's nothing about them that's not cancer. It just hasn't developed fully," Dr Oubre explains.

Stage I and II cancer

In these stages, the cancer is generally in only one area of the body. Stage I cancer is usually smaller in size and stage II is usually larger in size.

Stage III

During this stage, it often means that the cancer has spread to the lymph nodes.

Stage IV

This stage typically indicates that the cancer has spread widely throughout the body or has metastasised.

3 common questions about cancer staging

Does TNM coincide with cancer stage?

It's important to note that the combination of TNM categories you have doesn’t necessarily coincide with the stage of cancer you have.

"You might have a T3, N0, M0 colon cancer, and that would be stage II. But a T3, N0, M0 of cancer of a different type might be stage I," says Dr Oubre.

There are other factors that affect the stage of cancer. For example, the grade of your cancer measures how abnormal the cancer cells look. Low-grade cancers look similar to cells in normal tissue and grow slowly while high-grade cancers grow rapidly. Cell type also helps determine the stage of cancer because your treatment will depend on the type of cancer cell.

Do cancer stages change over time?

Interestingly, the stage of cancer that you have never changes over time, even if it metastasises. It remains whatever it was called at the time of diagnosis.

So, if you were diagnosed with stage II breast cancer and went into remission, but it came back and spread to your bones, it wouldn’t be called stage IV. It would be called stage II breast cancer with recurrent disease in the bones.

Is one person's stage III cancer the same as another person's stage III cancer?

With all of this said, keep in mind that all cancers at different stages have different prognosis and treatment options. So someone with stage III for one type of cancer doesn’t necessarily have the same treatment or prognosis as another type of cancer at the same stage.

"Patients often come with memories or accounts of how their friend/family coped with their cancer treatments. I always remind patients that not all cancers behave the same way," says Dr Hou. "And advancements in cancer treatments differ across different cancer types."

That's why you should always discuss your diagnosis in depth with your doctor to make sure that you understand what it means for you and your treatment.

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