What is breast cancer?

Breast cancer is a malignant tumour which starts in the breast tissue. Breast cancer is the most common cancer in New Zealand women. The majority of breast cancers begin in the milk ducts (ductal cancers), while a small number start in the milk sacs or lobules (lobular cancers). Within these two groups there are different subtypes of breast cancer.

Some breast cancers 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.

About your breasts

Your breasts are designed to make milk after pregnancy. The breast tissue extends almost to the collar bone at the top and to the armpit at the side. Lying beneath the breasts are the chest muscles and ribs.

Breast tissue is made up of milk glands, connective tissue and fat. The milk glands consist of milk sacs (lobules) where milk is made, and ducts which take the milk to the nipple.

In your breast area and armpit there are lymph glands or nodes which are connected by a system of lymph ducts. These glands and ducts are part of the lymph system which helps your body to fight infection.

Your breasts do not stay the same throughout your adult life. Your monthly period, pregnancy, age and weight changes can all alter their shape. Some women find their breasts feel more tender and lumpy before their period. This is quite normal.

How many people get breast cancer?

In New Zealand, about 2300 women are diagnosed each year with breast cancer. Breast cancer can occur at any age, but is most common in women between the ages of 50 and 70 years.

Although it is very unusual, men can develop breast cancer (approximately 1% of breast cancers).

What causes breast cancer?

The causes of breast cancer are not clear, so there is no certain way to prevent it. There are some clues, or risk factors, about who is more likely to develop the disease.

The most important risk factors are:

• age - a woman's chances of developing breast cancer increase as she gets older
• previous breast cancer
• 'atypical hyperplasia' (the milk ducts contain increased numbers of abnormal cells) can be seen in a breast biopsy
• a family history of breast cancer.

Women with a family history of breast cancer may have an increased risk of getting breast cancer. The risk can be mild, moderate or high and depends on the number of relatives affected, whether they are first or second degree relatives, and the age of the relative(s) when their breast cancer was found. 

However, even among women with a high risk, most will not develop breast cancer. Those women with a very strong family history of breast cancer, who are shown to have inherited one of the abnormal genes associated with breast cancer, have an increased risk of developing breast cancer. Most women who develop breast cancer have no family history of the disease.

Related topics

Also see Breast cancer diagnosis and Breast cancer treatment and Breast screening: mammograms

What does that term mean? See Cancer glossary

Original material provided by the Cancer Society, June 2007. Reviewed by everybody, July 2010.