A mammogram is a specialized type of X-ray that examines the breast and breast tissue. While there are several methods of performing a mammogram, the doctor who interprets a mammogram is looking for several characteristics of breast tissue that could indicate the presence of cancer. Depending upon age, menopausal status and breast size, normal breast tissue may appear on the mammogram as dense tissue (white on a mammogram). This tissue is often composed of normal fibrous tissue or dense tissue consisting of breast or mammary glands. On the other hand, fatty tissue, a larger component of breast tissue, appears black. Dense breast tissue on a mammogram can be described with several terms, including dense tissue, fibrous changes, or apparently in your case, a density presumably composed of fibrous and glandular tissues.
The difficulty arises in interpreting the cause of dense white breast tissue on a mammogram because cancer can also appear as a white area. If the white area appears in the background of surrounding fatty tissue, the interpretation of a possible cancer is more easily considered. If the cancer is surrounded by dense, fibrous tissue, the person interpreting the mammogram cannot distinguish the actual composition of that white area, which may also contain a cancer. To further define the specific dense area, an ultrasound is often recommended in addition to the mammogram. This test can further distinguish whether the dense area has fluid in it and can help define a lump within the white area. Experienced mammographers can then better determine the likelihood that the density is benign or more suspicious for cancer.
The only way to be absolutely sure about the tissue composition (cancer or benign) of a dense abnormality on mammogram is to have a biopsy.