TNM Cancer Staging Explained: What T, N, M and Stages I–IV Mean

TNM staging is a short way of recording how large the tumour is (T), whether it has reached the nearby lymph nodes (N) and whether there are distant metastases (M). The combination of these three letters gives a stage from I to IV. The stage largely shapes the goal of treatment, but on its own it does not determine the outlook — especially when you compare different cancers.

What TNM is and where it comes from

TNM is an international system maintained by two bodies: the Union for International Cancer Control (UICC) and the American Joint Committee on Cancer (AJCC). The rules for each type of cancer are set out in their manuals. Many reports today refer to the 8th edition (UICC published it in 2016). In 2025 UICC released the 9th edition and recommended using it from 1 January 2026; the changes affect certain sites (for example, the lung, prostate and oropharynx), while the general principle stays the same.

So if your documents say “TNM 8” or “AJCC 8th”, that’s normal. What matters is that your doctor knows which edition was used, if different documents in your records use different ones.

What T, N and M mean

T (tumour) — the primary tumour.

• TX — the primary tumour cannot be assessed.
• T0 — no primary tumour is found.
• Tis — carcinoma in situ (the cancer has not grown beyond the layer of cells where it started).
• T1–T4 — the size of the tumour and/or how far it has grown into nearby tissues. The higher the number, the larger the tumour or the deeper the growth. There are subcategories: T1a, T3b and so on.

N (nodes) — regional lymph nodes.

• NX — the lymph nodes cannot be assessed.
• N0 — no cancer in the regional lymph nodes.
• N1–N3 — lymph nodes are involved; the number reflects how many and/or where they are.

M (metastasis) — distant metastases.

• M0 — no distant metastases.
• M1 — the cancer has spread to distant organs or tissues.

What T2 or N1 actually means is specific to each cancer. T2 in breast cancer and T2 in stomach cancer describe quite different situations: in one case size is what matters, in the other it is how deeply the tumour has grown into the stomach wall.

The c, p, y and r prefixes: what kind of assessment it is

Small prefixes often appear before the letters. They show what the assessment is based on:

The clinical and pathological stages may differ — this is common, not a mistake. Surgery simply gives more information than scans. I explain how to read pT and pN in a pathology report in my article on pathology reports.

The y prefix shows what was left of the tumour after treatment. For example, “ypT0 ypN0” means no cancer cells were found in the removed tissue.

How TNM becomes stage I–IV

Combinations of T, N and M are grouped into stages:

• Stage 0 — carcinoma in situ; the cells have not spread beyond their own layer.
• Stages I–III — there is a tumour but no distant metastases. The higher the number, the larger the tumour or the wider the involvement of lymph nodes and nearby tissues.
• Stage IV — there are distant metastases (M1).

Letters after the number (IIA, IIIB) refine the group.

For some cancers, the stage includes more than T, N and M. In breast cancer, for example, the AJCC 8th edition uses a so-called prognostic stage, which also takes into account tumour grade, hormone receptors and HER2. That’s why one discharge summary may contain two different stages — an anatomical one and a prognostic one. I explain how to read these markers in my article on IHC in breast cancer.

And not every cancer is staged with TNM. Lymphomas use other systems, gynaecological cancers use the FIGO system, which is close to TNM, and most brain tumours have no formal stage at all.

Does the stage change over time?

No. The stage assigned at diagnosis stays with you. If the tumour shrinks after treatment, your documents will not show a “lower stage”: the result is recorded separately with the prefix y. If the cancer comes back or spreads, a new assessment is added with the prefix r, but the original stage is not rewritten.

In conversation, doctors sometimes say “it has become stage four” when metastases appear. Strictly speaking that’s inaccurate, but the meaning is clear: the disease has become advanced and the goal of treatment is reconsidered.

Why stage 4 is not a death sentence — and not one single thing

Stage 4 means distant metastases. It does not mean treatment is pointless, and it does not tell you how much time is left.

First, stage 4 in different cancers means different diseases, with different courses and different treatment options. Differentiated thyroid cancer shows this well: under the AJCC 8th edition, patients younger than 55 are classed as stage II even with distant metastases — that’s how differently this cancer behaves.

Second, within one type of cancer a great deal depends on its biology: receptors, mutations, markers. At the same stage IV, patients with different molecular profiles receive different treatment and have different results. That’s why, besides the stage, a doctor always looks at the pathology, IHC and, if needed, molecular tests.

Third, “metastases” is not one condition. A single metastasis and widespread involvement of several organs are different clinical situations, and for some cancers with limited spread the discussion includes not only drugs but also surgery or radiotherapy to the metastases.

How the stage affects the goal of treatment

The stage is one of the main guides when choosing a strategy, but not the only one. The American Cancer Society states plainly that stage is not the only factor: the type of cancer, its characteristics and the person’s health are also taken into account.

In simplified terms, the logic is:

• Early stages (I–II). The goal is usually cure. Treatment is based on local therapy (surgery, radiotherapy), and drug treatment before or after it is added when it lowers the risk of the cancer coming back.
• Locally advanced disease (often III). The goal is usually still cure, but treatment is more often combined: drug therapy, surgery and radiotherapy in a set sequence. More on this in my article on treatment before and after surgery.
• Advanced disease (IV). Treatment is most often based on systemic therapy: chemotherapy, targeted therapy, hormone therapy or immunotherapy. The goal is to control the disease for as long as possible and preserve quality of life; for certain cancers and situations, a more radical approach is also discussed.

Questions to ask your oncologist

• What is my stage — clinical or pathological, and which edition of TNM was used?
• Have all the tests needed for staging been done, or is something else required?
• What is the goal of treatment in my case: cure or disease control?
• Which characteristics of the tumour, besides the stage, affect the choice of treatment?
• Will the plan change if the stage turns out to be different after surgery?

If the stage in your discharge summary is unclear, or differs between documents, I’ll go through your documents in a consultation and explain what it means for your treatment.

Sources

1. Cancer Staging — National Cancer Institute (NCI), 2022
2. Cancer Staging — American Cancer Society, 2024
3. TNM Classification of Malignant Tumours, 8th edition — UICC, 2016
4. 9th edition of the UICC TNM Classification of Malignant Tumours now available — UICC, 2025
5. Thyroid Cancer Stages — American Cancer Society, 2024
6. AJCC 8th Edition Breast Cancer Staging — American College of Surgeons / AJCC, 2018