What is a margin?

Any tissue that needs to be cut by a surgeon in order to remove an abnormal area of tissue such as a tumour from the body is called a margin. The margin is usually made up of a small rim of normal, healthy tissue. The purpose of a margin is to ensure that all abnormal cells, even those at the very edge that may be hard for the surgeon to see, are removed at the time of surgery.

How big does a margin need to be?

The size of the margin (the distance between the abnormal tissue and the cut edge) depends on the type of tumour being removed and the location in the body. For example, when a cancer is removed it is often very difficult to tell where the abnormal cells stop and the normal cells begin. For that reason, surgeons will usually take a large or ‘wide’ margin in order to be sure that  they have removed all of the abnormal cells.

In some areas of the body, however, it is very difficult to take a large margin even for malignant (cancerous) tumours because there is very little normal tissue around it. Benign (non-cancerous) tumours on the other hand often have very clear borders and may be removed with only a very small or ‘thin’ margin of normal tissue around them.

Positive and negative margins

By examining the margin tissue under a microscope, your pathologist can see how far the tumour  cells are from the edge of the normal, healthy tissue. If tumour cells are seen at the cut edge of the tissue, the margin is called ‘positive’. If the tumour cells are away from the cut edge of the tissue, the margin is called ‘negative’.

The status of the margin (whether it is ‘positive’ or ‘negative’) is a very important part of the pathology report. A positive margin means that some tumour cells may have been left in the body when the tumour was removed. As a result, patients with a positive margin may be offered additional treatment to prevent the tumour from re-growing.

Margins examined during your surgical procedure

In some cases, a surgeon may send tissue samples called “margins” for examination by a pathologist while the patient is still in the operating room. This situation is called an intraoperative consultation or a frozen section. These specimens are called margins because the tissue sample has been taken from an area that the surgeon believes is outside of the abnormal area or tumour.

Intraoperative consultations provide a rapid pathological diagnosis which allows the surgeon to make changes to the surgical plan if necessary. For example, if one of the margins examined during an intraoperative consultation is called ‘positive’, the surgeon may take additional tissue from around the tumour in order to make sure that no tumour cells are left in the body.

Not all tissue samples have margins

Most small tissue samples such as biopsies do not include a margin because these types of procedures are not performed with the purpose of removing the entire tumour.

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