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 negative margin means that no cancer cells have been found in the cells at the outer edge of the surgically-removed tissue. A negative margin is one of the terms that is slightly ambiguous, due to being perceived differently by various pathologists. Some pathologists label a tumor’s margin “negative” if even one normal cell is found between the cancer cells and the outer edge of the surgically-removed tissue. Other pathologists believe that there most be at least 2 millimeters between the cancer cells and the outer edge of the tissue for a margin to be labeled “negative.”