edge of the tissue removed during surgery. A negative surgical margin means that no cancer cells were found on the outer edge of the removed tissue, and is considered a sign that none of the cancerous mass was left behind. A positive surgical margin means that cancer cells are found at the outer edge of the tissue removed and is usually a sign that some cancer remains in the body. Sometimes the actual edge looks clear but the cancer cells are very close to the edge, called a narrow surgical margin. With certain aggressive types of cancer, a narrow margin is reason enough for another surgery to remove more tissue.