When a rectal cancer is removed, the piece of bowel containing the cancer is taken away together with some apparently normal bowel either side of it. This cylinder of tissue is then sent to the pathologist who examines it under the microscope.

In the past surgeons and pathologists have taken great care to make sure that the cut end of the bowel (at the top and bottom of the tissue removed during the operation) is free from any trace of cancer. If the checks under the microscope confirm this then this shows that the top and bottom margins of the tissue are clear.

In recent years doctors have realised that it is also very important to check and be sure that the edges of the tissues surrounding the bowel which has been removed are also free from tumour. This means that careful checks are made that the margins of the operation tissue all round the side of the cancer are clear as well as the margins above and below the growth.

Because of this new understanding of how important it is to make sure tumour is clear from edges around the growth, as well as at either end of the cut bowel, the type of surgery that is done for a rectal cancer has changed. Wherever possible the surgeon will take away a complete cylinder of tissue containing the tumour, some normal bowel above and below the growth and a circle of surrounding normal fatty tissue, including the tissue between the rectum and the back of the pelvis. This is called a total mesorectal excision (TME).

This newer type of operation, taking away the extra collar of normal fatty tissue around the growth, has been shown to reduce the risk of the cancer coming back and so is now increasingly recommended.

If your surgeon has said that the examination under the microscope after your operation showed that the margins were clear, he means that the pathologist looked at the top, the bottom and the all round the side edges of the tissue that was removed and found no tumour at those edges. This means that there is an excellent chance that your growth has been completely taken away.

Although the fact that tumour margins are clear is very encouraging and substantially reduces the risk of the cancer coming back in the region around the rectum; it does not guarantee that the cancer has not spread to other places before it was removed. Other tests, such as looking at the lymph nodes and blood vessels in the tissue that were taken away will give your doctors some idea of whether there is a substantial risk that spread elsewhere might have occurred. If so, your doctor may discuss using chemotherapy to reduce the risk of the cancer coming back.