I know this is an old post however I think should be given an answer:

Fibrocystic breasts are characterized by lumpiness and usually discomfort in one or both breasts. The condition is very common and benign, meaning that fibrocystic breasts are not malignant (cancerous). Fibrocystic breast disease (FBD), now referred to as fibrocystic changes or fibrocystic breast condition, is the most common cause of "lumpy breasts" in women and affects more than 60% of women. The condition primarily affects women between the ages of 30 and 50 and tends to become less of a problem after menopause.

The diagnosis of fibrocystic breasts is complicated by the fact that the condition can vary widely in its severity. In some women, the symptoms of fibrocystic breast condition can be very mild with minimal breast tenderness or pain. The symptoms can also be limited in time, usually occurring only premenstrually. It may not even be possible to feel any lumps when the breasts are examined by the woman herself or by her doctor. In other women with fibrocystic breasts, the pain and tenderness are constant, and many lumpy or nodular areas can be felt throughout both breasts.

Is there a difference between fibrocystic breast condition and fibrocystic breast disease?

No. In the past, fibrocystic breast condition was often called fibrocystic breast disease. However, it is not a disease, but a condition. Most women tend to have some lumpiness in their breasts. Therefore, it is now being more appropriately termed fibrocystic breast condition. The abbreviation is FCC (an acronym derived from FibroCystic breast Condition).

Other names that have been applied to fibrocystic breast condition include mammary dysplasia, chronic cystic mastitis, diffuse cystic mastopathy, and benign breast disease (a term that includes other benign breast disorders, including infections).

What causes fibrocystic breasts?

Fibrocystic breast condition involves the glandular breast tissue. The sole known biologic function of these glands is the production, or secretion, of milk. Occupying a major portion of the breast, the glandular tissue is surrounded by fatty tissue and support elements. The glandular tissue is composed of different types of cells: (1) clusters of secretory cells (cells that produce milk) that are connected to the milk ducts (tiny tubes); and (2) the cells that line the surfaces of the secretory cells, called the epithelial cells.

The most significant contributing factor to fibrocystic breast condition is a woman's normal hormonal variation during her monthly cycle. Many hormonal changes occur as a woman's body prepares each month for a possible pregnancy. The most important of these hormones are estrogen and progesterone. These two hormones directly affect the breast tissues by causing cells to grow and multiply.

Many hormones aside from estrogen and progesterone also play an important role in causing fibrocystic breasts. Prolactin, growth factor, insulin, and thyroid hormone are some of the other major hormones that are produced outside of the breast tissue, yet act in important ways on the breast. In addition, the breast itself produces hormonal products from its glandular and fat cells. Signals that are released from these hormonal products are sent to neighboring breast cells. The signals from these hormone-like factors may, in fact, be the key contributors to the symptoms of fibrocystic breast condition. These substances may also enhance the effects of estrogen and progesterone and vice versa.

The same cyclical hormones that prepare the glandular tissue in the breast for the possibility of milk production (lactation) are also responsible for a woman's menstrual period. However, there is a major difference between what happens in the breast and uterus.

In the uterus (the womb), these hormones promote the growth and multiplication of the cells lining the uterus. If pregnancy does not occur, this uterine lining is sloughed off and discharged from a woman's body during menstruation.

In the breast, these same hormones stimulate the growth of breast glandular tissue and increase the activity of blood vessels, cell metabolism, and supporting tissue. All this activity may contribute to the feeling of breast fullness and fluid retention that women commonly experience before their menstrual period.

When the monthly cycle is over, however, these stimulated breast cells cannot simply slough away and pass out of the body like the lining of the uterus. Instead, many of these breast cells undergo a process of programmed cell death, called apoptosis. During apoptosis, enzymes are activated that start digesting cells from within. These cells break down and the resulting cellular fragments are then further broken down by scavenger cells (inflammatory cells) and nearby glandular cells.

During this process, the fragments of broken cells and the inflammation may lead to scarring (fibrosis) that damages the ducts and the clusters (lobules) of glandular tissue within the breast. The inflammatory cells and some of the breakdown fragments may release hormone-like substances that in turn act on the nearby glandular, ductal, and structural support cells.

The amount of cellular breakdown products, the degree of inflammation, and the efficiency of the cellular cleanup process in the breast vary from woman to woman. These factors may also fluctuate from month to month in an individual woman. They may even vary in different areas of the same breast in a woman.

~Zig