The stage of a breast cancer is a measure of how far it has progressed. So, a cancer that is small and confined to the breast is at an early stage, whereas one that has spread to many different parts of the body is at an advanced stage.

Over the years doctors have worked out staging systems for all the different types of cancer. Some of these are quite simple whilst others are very complicated. Rather confusingly there are several different types of staging system for breast cancer. Although these vary in their details (and complexity) they all follow a similar pattern. One of the oldest, and simplest, staging systems for breast cancer has four stages, these are:

• Stage 1 - the earliest stage, when the cancer is only in the breast and does not affect the skin overlying the breast
• Stage 2 - the next stage, when the cancer has spread to the lymph nodes under the arm
• Stage 3 - this is when the cancer involves the skin of the breast (whether or not it has spread to the lymph nodes under the arm)
• Stage 4 - this is when the cancer has spread to other parts of the body.

Like the example above, some staging systems for breast cancer use the numbers 1 to 4 for the various stages (often subdividing each stage into a, b or c), whilst another system uses the initial T, N and M, followed by a number to describe the primary tumour (T), any involved lymph nodes (N) and the presence or absence of any spread to other organs, or metastases (M).

Doctors use the results of tests and operation findings to decide the stage of a woman's breast cancer. Sometimes the stage will change as more results become available. For example, a woman with breast cancer might have examinations and mammograms which suggest the tumour is confined to the breast (stage 1) but when an operation is done, and the tissues examined under a microscope, tiny seedlings of tumour may be found in the lymph glands under the arm, making it a stage 2 cancer.

Using a staging system has two main benefits. Firstly it gives an idea of the size of a tumour and whether it has spread at all and so helps predict what the likely outcome of treatment will be. Secondly, 'staging' the cancer determines what the treatment should be, since often the treatment for an early stage cancer will be different from that for a more advanced tumour.

The grade of a cancer refers to the appearances of the tumour under the microscope. Depending on the appearances breast cancers may be given one of three grades.

• A 'low' grade is where the breast cancer cells look very like normal breast cells, with only slightly abnormal changes (these cancers are called 'well-differentiated').
• A 'high' grade is where the cells look very abnormal and show little or no resemblance to normal breast tissue (these cancers are called 'poorly differentiated').
• An 'intermediate' grade is somewhere between the high and low grades (these cancers are called 'moderately differentiated').

For some cancers, including breast cancer, the three different grades are often given numbers, so a low grade breast cancer is called Grade I, whereas a high grade breast cancer is called Grade III.

The grade of a breast cancer is considered to be a guide to how aggressive the tumour is and how likely it is to spread: an intermediate grade cancer is likely to be more aggressive than a low grade and a high grade cancer is likely to be more aggressive than an intermediate grade tumour.

Taken together the stage and grade of a breast cancer do help doctors to predict how that cancer might behave, how it might respond to treatment, and what the chance of cure might be. But the stage and grade are only guides to what might happen and breast cancers do not always behave in the way that might be expected from their stage and grade.