Breast cancer staging

The TNM system evaluates anatomic factors which can help determine the likely outcome of the disease (prognosis) and select an appropriate treatment plan.

These factors are:

• T - the size of the tumour
• N - the number and location of cancerous lymph nodes
• M - the presence or absence of metastases (secondary cancer spread from the original primary site).

T refers to tumour size

Size is measured under a microscope. It is important because the smaller the tumour, the less likely it is to have spread beyond the breast and the better the chance for successful treatment. Small tumours are generally classified as two centimetres (20mm) or less.

N refers to lymph node status

Lymph node status is determined by a pathologist who examines any nodes under a microscope after they have been removed from the armpit (axilla) during surgery. If any nodes contain cancer cells, they are called positive nodes.

M refers to metastasis

Metastasis is the spread of cancer to other areas of the body, otherwise known as distant spread. Some breast cancer cells have the ability to invade lymphatic and/or blood vessels where they can circulate to distant organs and tissues e.g. bones, liver, lungs and brain. Imaging such as CT scans and bone scans can be performed if the tumour is high risk and/or metastases are suspected.

Until recently breast cancer staging used only these measures to classify the stage of the cancer but in January 2018 the system was updated to take into account additional tumour biology factors that can affect outcomes.

These are:

• tumour grade
• hormone receptor expression
• HER2 over-expression
• the results of genomic tests (if used).

Using this updated system, the extra information provided by these biologic factors may modify the stage that would have been assigned by using anatomic factors alone. For instance a small but high-grade (grade 3) tumour that is hormone receptor positive and HER2 negative would be assessed as one stage higher than previously. Similarly a tumour that is triple negative, regardless of grade, would be determined to be one stage higher, or a larger tumour with a good biologic profile might be downstaged.

Results from predictive genomic panels such as Oncotype Dx may in some cases downgrade tumours by one stage compared to the old system using TNM alone if the results show a low score for risk of recurrence.

The TNM scale ranges from stage 0 to stage IV. A higher stage indicates more advanced cancer. The chart below gives a summary of TNM staging.

Stage should not be confused with Grade (1, 2 or 3), which reflects what the cancer cells look like under a microscope, how different they look from normal healthy breast cells and how actively they are dividing and growing.

Examples of Revisions to Breast Cancer Staging Using Biomarkers and Oncotype DX

Abbreviations:

• −, negative
• O+, positive
• ER, estrogen receptor
• G, grade
• HER2, human epidermal growth factor receptor 2
• M, metastasis classification
• N, lymph node classification
• PR, progesterone receptor
• T, tumor classification
  

Taken from the American Joint Committee on Cancer 2017 (8th edition)