Why staging is used

Staging describes the size of a cancer and how far it has spread outside of the lung. It helps healthcare teams plan the most appropriate treatment. Stage is only assessed at the time of diagnosis, although different systems may be used.

Staging is usually based on imaging tests, such as computed tomography (CT), positron emission tomography (PET), or magnetic resonance imaging (MRI), and sometimes biopsy results. Not all staging systems are used for everyone.

Limited- and extensive-stage disease

Small cell lung cancer (SCLC) is often classified as either limited or extensive stage:

Limited stage means that the cancer is found on one side of the chest and can safely be treated with radiotherapy to that area.
Extensive stage means that the cancer has spread beyond one side of the chest, for example to the other lung or elsewhere in the body.

TNM staging

TNM is a more detailed staging system used across many different types of cancer.

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 range from 1 to 4 and are based on the TNM categories.

In general, lower stage numbers mean the cancer is smaller and has not spread very far.

In small cell lung cancer, limited disease is normally either Stage 1, 2, or 3. Extensive small cell lung cancer normally means Stage 4.

Staging is important, as it helps with decisions about the most appropriate treatment and is also an indicator of the aggressiveness of the disease.

References

National Comprehensive Cancer Network® (NCCN®). NCCN guidelines for patients: Small cell lung cancer, 2024. https://www.nccn.org/patients/guidelines/content/PDF/SCLC-patient-guideline.pdf. Accessed Mar 3, 2026.
Cancer Research UK. Stages of lung cancer. https://www.cancerresearchuk.org/about-cancer/lung-cancer/stages-types/stages. Updated Oct 6, 2025. Accessed Mar 3, 2026.
Cancer Research UK. Limited and extensive stage (small cell lung cancer). https://www.cancerresearchuk.org/about-cancer/lung-cancer/stages-types/limited-extensive#tnm-and-number-staging-systems. Updated Jan 13, 2026. Accessed Mar 3, 2026.