The breast is a gland that consists of breast tissue supported by connective tissue (flesh) surrounded by fat. The easiest way to understand how the inside of the breast is formed is by comparing it to an upturned bush. Its leaves are known as lobules and they produce milk that drains into ducts that are the branches of the breast tree. These in turn drain into 12 or 15 major or large ducts which empty onto the surface of the nipple, just like the branches of a tree drain to the trunk.

Breast cancer develops from the cells that line the breast, lobules and the draining ducts. Cancer cells that remain confined to the lobule and the ducts are called 'in situ' or 'non-invasive'. They are sometimes also referred to as pre-cancers in recognition of the fact that these cells have not yet gained the ability to spread to other parts of the body, which is the feature that most people associate with cancer. An invasive cancer is one where the cells have moved outside the ducts and lobules into the surrounding breast tissue.

• Generally, breast cancers are not painful and women do not feel unwell with them.
• Breast cancer is now commonly diagnosed by breast screening in women who have no symptoms. Approximately 6 in every 1000 women between the ages of 50 and 64 who attend for screening will be found to have breast cancer the first time they attend screening.
• A lump in the breast. In many cases, the woman herself will first suspect the disease because she notices a lump or an area of lumpiness or irregularity in her breast tissue. This may happen when she is examining her breasts or while washing or applying lotion to her breasts, or the lump may be visible.

Other signs of breast cancer include:

• a change in the skin: there is often dimpling or indentation of the skin with the formation of wrinkles. The nipple might be pulled in or there may be a discharge from the nipple.
• occasionally the nipple itself changes. A rash can affect the nipple or the nipple may weep.
• the breast may swell and become red and inflamed or the skin may change and become like the skin of an orange. In some breast cancers this is due to a blockage of the drainage of fluid from the breast.
• patients sometimes present with a lump under the arm which is a sign that the cancer has spread to the lymph glands.

Age

The incidence of breast cancer increases with age and doubles every 10 years until the menopause when the rate of increase slows. Approximately a quarter of breast cancers affect women under the age of 50, a half occur between the ages of 50 and 69 and the remaining quarter develop in women who are 70 years or older.

Geographical variation

There is quite a difference in incidence and death rate of breast cancer between different countries. The biggest difference is between Eastern and Western countries. Recent, age-adjusted figures show that the rate of breast cancer per 100,000 women is 24.3 in Japan and 26.5 in China compared to 68.8 in England and Wales and 72.7 in Scotland and 90.7 in North America in white females.

However, studies of women from Japan who emigrate to the US show that their rates of breast cancer rise to become similar to US rates within just one or two generations, indicating that factors relating to everyday activities are more important than inherited factors in breast cancer.