What is breast cancer?

Breast cancer is a malignant tumour that starts in the breast tissue. The majority of breast cancers begin in the milk ducts (ductal cancers). A small number start in the milk sacs or lobules (lobular cancers). Within these two groups there are different subtypes 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, most commonly the lung, bones, and liver.

A breast showing ductal carcinoma in situ and invasive breast cancer

How common is breast cancer?

Breast cancer is the most common cancer in New Zealand women. Approximately 2,500 women are diagnosed each year. Breast cancer can occur at any age but is most common in women between the ages of 50 to 70 years. Although it is very unusual, men can develop breast cancer (approximately 1 percent of all breast cancer). For information on male breast cancer contact your local Cancer Society, phone the Cancer Information Helpline 0800 CANCER (226 237) for a copy of our booklet Breast Cancer in Men: From one man to another. This booklet can also be viewed on the Cancer Society’s website.

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 risk factors include:

  • age – a woman’s chances of developing breast cancer increase as she gets older

  • previous breast cancer

  • atypical hyperplasia (increased number of abnormal cells) can be seen in breast biopsy

  • a family history of breast cancer

  • having a faulty BRCA gene. The BRCA1 gene when working normally helps to repair DNA

  • having an altered gene that is associated with the risk of breast cancer

  • hormone replacement therapy

  • alcohol consumption

  • obesity.

The risk from family history depends on:

  • the number of relatives affected

  • whether they are close relatives

  • the age of the relative(s) when their breast cancer was found.

However, a family history of breast cancer does not necessarily mean a woman will develop breast cancer.

Women who are shown to have inherited one of the faulty genes (for example BRCA) associated with breast cancer do have an increased risk. If it seems possible that you may be a member of a family at increased risk, you will be referred to a family cancer genetic clinic.

Most women who develop breast cancer have no family history of the disease.