Why staging is used

Staging describes the size of a cancer and how far it has spread at the time of diagnosis. It helps healthcare teams work out which therapies are likely to be most effective and guides discussions about prognosis. It is only assessed at diagnosis.

TNM staging

TNM is a staging system commonly used in non-small cell lung cancer (NSCLC) and many other cancers.

T (Tumor) – the size of the tumor.
N (Node) – whether lymph nodes are affected.
M (Metastasis) – whether the cancer has spread to other parts of the body.

In TNM staging, the cancer is given a number for each of the three letters. Lower numbers usually mean that the cancer is smaller or has not spread as far. For example, T1N0M0 describes a small tumor in the lung that has not spread to the lymph nodes or anywhere else in the body. T4N3M1a may describe a larger tumor (bigger than 7 cm) that has spread to lymph nodes further away and to the opposite lung.

TNM staging can also include additional numbers or letters to give more detail about the size of the tumor and where it has spread.

Number staging

Number stages are based on TNM categories. Most of the time, non-small cell lung cancer is Stage 1, 2, 3 or 4. In general, lower stage numbers mean the cancer is smaller and has not spread very far.

Stage 1: The tumor is small (4 cm or smaller) and only in one lung – it has not spread to the lymph nodes.
Stage 2: The tumor may be larger, may have grown into nearby lung tissue, or may have spread to nearby lymph nodes.
Stage 3: The cancer has spread further within the chest. It may involve lymph nodes in the space between the two lungs (the mediastinum) or on the opposite side of the chest, or it may have grown into nearby structures such as the chest wall, the diaphragm, or the lining around the heart.
Stage 4: The cancer has spread to the other lung or other organs. This stage is split into:
Stage 4a: When it has spread to just one other area outside the lung, such as the other lung or one place in another organ.
Stage 4b: When it has spread to multiple organs or areas outside the lung.

If non‑small cell lung cancer is found very early, it may be classified as occult stage or stage 0 (carcinoma in situ).

References

Cancer Research UK. TNM staging for lung cancer. https://www.cancerresearchuk.org/about-cancer/lung-cancer/stages-types/tnm-staging. Updated Jan 13, 2026. Accessed Mar 4, 2026.
National Cancer Institute. Non-small cell lung cancer treatment (PDQ®)–patient version. https://www.cancer.gov/types/lung/patient/non-small-cell-lung-treatment-pdq. Updated May 16, 2025. Accessed Mar 4, 2026.