When a breast tumor is removed, either by biopsy, lumpectomy, resection, or mastectomy, some of the surrounding breast tissue (i.e., termed the surgical margin) is also removed. The pathologist places the excised tissue in a special ink and examines the outermost edge of the tissue under the microscope to see whether or not the margin contain cancer cells, and how far from the edge the cancer cells are. This study allows the pathologist to determine whether or not all of the tumor was removed.
Doctors use the term margin or margins of resection to refer to the distance between the breast tumor and the edge of the tissue. The margins are measured on all six sides: front and back, top and bottom, left and right. Knowing how close the cancer cells are to the edge of the tissue will help your doctor decide upon the next step in your treatment, i.e.: additional surgery, radiation, etc.
A positive margin mean that there are cancer cells present at the outer edge of the surgically-removed tissue.