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. When a surgery is performed to remove a tumour, the surgeon usually attempts to remove the entire tumour plus a small rim of ‘normal’ tissue around it. In this type of case, the margin is the normal tissue.

The purpose of a margin is to ensure that all abnormal cells, even those at the very edge of the tumour that may be hard for the surgeon to see, are removed at the time of surgery.

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 (a malignant tumour) 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 tumours because there is very little normal tissue around it.

Non-cancerous (benign) 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.

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

The status of the margin (whether it is ‘positive’ or ‘negative’) is a very important part of the pathology report. A positive margin suggests that some tumour cells may have been left in the body when the tumour was removed. As a consequence, patients with a positive margin may be offered additional medical management, for example surgery or radiation to the site. 

In some cases, the surgeon may submit called “margins” for pathological examination 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 all of the tissue 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 allow the surgeon to make changes to the surgical plan if necessary. For example, if a margin specimen is submitted for intraoperative consultation and the pathologist sees tumour cells in the tissue (a positive margin), the surgeon may take additional tissue from around the tumour in order to make sure that no tumour cells are left behind.

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

Example of a negative tumour resection margin viewed under the microscope

This image is from a prostate resection specimen. The cancer cells can be seen in the upper left corner of the image (white arrow). The margin has been painted with ink  (black arrow). The tumour cells are quite far from the edge of the tissue making this a ‘negative margin’.

Example of a positive tumour resection margin viewed under the microscope

Postive margin

This image is from a skin resection specimen. The margin has been painted with green ink (black arrow). The cancer cells (white arrow) have also been painted with green ink and are right at the edge of the tissue, making this a ‘positive margin’.

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