What is Breast Cancer?

All Cancers are a disease of the body's cells. Normally, the body's cells grow and divide in an orderly way, allowing your body to grow and to heal after an injury. Occasionally, some cells behave in an abnormal way and grow into a lump called a tumour. Tumours can be benign (not a cancer) or malignant (a cancer). Benign tumours do not spread to other parts of the body.  Breast Cancer is a malignant tumour which starts within the breast tissue. The majority of breast cancers begin in the milk ducts (intraductal cancers). A small number start in the milk sacs or lobules (lobular cancers). Within these two groups there are different types of breast cancer. Some grow very slowly, others develop more rapidly. Breast cancer can spread to the lymph glands and to other parts of the body such as the bones and liver.

Apart from skin cancer, breast cancer is the most common cancer in women. One in nine New Zealand women will develop breast cancer during their lives. Around 3000 women and 25 men are diagnosed each year.  Breast cancer is New Zealand’s third most common cancer and accounts for more than 600 deaths every year.  The risk of being diagnosed with breast cancer increases with age.  Breast cancer isn’t common in women under the age of 50.  About 70% of women who are diagnosed with breast cancer are 50 years or older.

Some women are at greater risk of breast cancer because there is a history of close family members having the disease. However, most women who develop breast cancer have no relatives with the disease. Even among women who do have relatives with breast cancer, most will never develop it.

What Causes Breast Cancer?

The cause of breast cancer is unknown. However, it is most unlikely that there is one single cause. A number of factors, some known and many unknown, probably work together to trigger its growth. Breast cancer usually occurs in women over 50 or for male breast cancer it occurs in men over the age of 65.  All women are at risk as they grow older. Although breast cancer does occur in women under 50, increasing age is the main factor influencing the risk of getting breast cancer. The factors listed below also seem to put women at slightly higher risk. However, they explain only a small number of breast cancer cases.

Women with a strong family history of breast cancer have an increased chance of getting breast cancer and a 60% increased risk of ovarian cancer. The increase in risk can be very small to quite large (only 5-10% of all breast cancers are hereditary), depending upon the number of relatives affected, the age of the relatives when their breast cancer was found, and the type of breast cancer they had.

Researchers are looking into a number of factors which may influence the development of breast cancer. Because breast cancer is more common in certain countries, lifestyle factors, such as diet are thought to be important. (Did you know that two glasses of wine each day could give you an increased risk of 41%?). Women who have already had breast cancer do have a slightly higher chance of a new breast cancer developing in their other breast. 

Hormone changes linked with pregnancy and menopause also appear to have an effect. None of these factors are certain enough though, to predict who will develop breast cancer. Risks linked with the pill suggest a possible increased risk of 10% and women on HRT can have an increased risk of up to 35%.