28 February 2023

Check Up #12 - Cancer staging

What is it and how does it work?

Check Up #12 - Cancer staging. What is it and how does it work?

When we hear about someone having been diagnosed with “stage IV” cancer, most of us know this is very bad news. It is the most advanced overall cancer stage.

Classifying – or staging – a cancer is paramount to determining the cancer’s prognosis (its likely evolution) and to choose the most appropriate treatment. A misclassified cancer can lead to wrong treatment options, including undertreatment or overtreatment.

To determine what the overall stage of a patient’s cancer is – stage I, II or III or IV –, it is necessary to start by considering separately the state of the primary (initial) tumour, the number of nearby lymphnodes that have been affected by the cancer and whether the disease is metastatic, which means that it has spread to distant sites in the body. 

For doing this, the most commonly used classification is the international TNM system. The letter T concerns the primary tumour; N tells about lymph nodes; and M whether metastases are present.

To classify the primary tumour, the letter T is followed by a letter or a number: TX means there is no information about the primary tumour, for instance because it can't be measured; T0 that there is no evidence of a primary tumour; Tis means in situ cancer (very limited growth). T1, T2, T3, or T4 are condensed descriptions of the tumour size and level of spread. The higher the number, the larger the tumour or the more deeply it has spread into nearby tissues, or both. Sometimes these classes can themselves be subdivided.

Similarly, for lymph node involvement, NX means there is no information about the nearby lymph nodes, or that they can’t be assessed; N0 that nearby lymph nodes do not contain cancer; and N1, N2, N3, N4 describe the size, location or number of nearby lymph nodes affected by the cancer. The higher the class number, the larger the spread to nearby lymph nodes.

Finally, the letter M can be followed by an X, 0 or 1: MX means the presence of metastases cannot be assessed; M0 that the cancer has not metastasized; M1 that it has. Once this classification has been performed, the cancer may be attributed codes such as T2N1M0, T4N3M1, and so forth.

The TNM system is used for most solid tumours. But brain and blood cancers, for instance, use other classification systems. Moreover, other factors, such as age, type of cancer cell, tumour markers for prostate cancer or hormone receptors for breast cancer may also have to be considered for staging.

Once the values for T, N, and M (and other factors) have been determined, they are combined into the overall cancer stage. For most cancers, the stage is coded as being I to IV (Roman numerals), plus a stage 0 if they are in situ. Stage 0 cancers can usually be entirely removed with surgery. Stage I cancers have not grown deeply into nearby tissues, and have not spread to the lymph nodes or other parts of the body. Also called early-stage cancers, they usually have a better prognosis than higher number stages. Stage II and Stage III cancers have grown more deeply into nearby tissue and may also have spread to lymph nodes, but not to other parts of the body. Stage IV cancers have spread to other parts of the body, and are also called advanced or metastatic cancers. Sometimes these stages can themselves be subdivided.

Doctors may also use words such as local, localised, regional, locally advanced, distant – or, as already mentioned, advanced or metastatic – to describe the cancer stage. Local and localised mean that the cancer is only in the organ where it started and has not spread to other parts of the body. Regional and locally advanced mean close to or around the organ. Distant, advanced and metastatic mean in a part of the body farther from the organ.

After treatment, a cancer can be restaged in the TNM system, for example if it has shrunk. But the cancer’s initial TNM staging does not itself change, because this would be confusing when looking at the cancer’s evolution with time and for estimating survival rates. Instead, a letter “r” will be written before the letter T, N, or M. Thus, a T2N1M0 cancer might be restaged as rT1N1M0.

Cancer staging is becoming more complex as new findings are added to staging systems for different types of cancer. But at the same time, this makes cancer staging ever more accurate and useful.



By Ana Gerschenfeld, Health & Science Writer of the Champalimaud Foundation.
Reviewed by: Professor António Parreira, Clinical Director of the Champalimaud Clinical Center.
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