Margin refers to the edges of a biopsy or other surgery. Usually, veterinarians attempt to remove the tumor plus a large area of what looks like normal tissue around it. By removing that normal tissue, they get a good “margin” or non-tumor area. This can help prevent spread. During a comprehensive margin evaluation, the pathologist examines the margin under a microscope to look for microscopic cancer cells. They also measure the margins. Wide margins of two to three centimeters are preferred because tumors with narrow margins are more likely to recur.