When a breast cancer is removed, the tumour is always sent to a pathologist who provides a detailed report about it. This report includes comments on the naked-eye appearance of the cancer, the appearances under the microscope and often the results of some chemical tests.

The microscopic examination of the tumour will give information about which type of cell within the breast the cancer developed from and whether or not there are any cancer cells in the blood capillaries or lymph vessels. The microscopic examination will also give the grading of the cancer.

Depending on the appearances, breast cancers are given one of three grades.

• Grade I is where the cancer cells look very like normal breast cells, with only slightly abnormal changes (these cancers are called 'well-differentiated'.

• Grade III is where the cells look very abnormal and show little or no resemblance to normal breast tissue (these cancers are called 'poorly differentiated').

• Grade II is an intermediate grade (these cancers are called 'moderately differentiated').

The grade of a breast cancer is considered to be a guide to how aggressive the tumour is: a grade II cancer is likely to be more aggressive than a grade I and a grade III cancer is likely to be more aggressive than a grade II.

The grade of the cancer is, however, only one of a number of factors that doctors need to consider when predicting the outlook for individual patients and deciding on the treatment to give the best possible chance of cure. Among other important factors are the size of the cancer and whether or not it has spread to involve the lymph nodes under the arm.