About Breast Cancer

In this section you can find out more about:

This will help to give you a better understanding of the disease. Remember, if you want to know more or you have further questions, note them down and speak to your medical professional.

What is Breast Cancer?

Breast cancer occurs when the cells in the breast begin to grow in an out-of-control fashion and form a tumour. Cancer cells can multiply much faster than normal cells and may spread to other parts of the body.

Breasts are made up of glandular tissue, connective tissue and fat, and each breast is closely associated with a network of lymph nodes and vessels (see image below). Breast cancers develop when a cell within breast tissue starts to grow and divide without the proper controls, producing more and more abnormal cells that form a tumour.


There are a number of different ways of classifying breast cancer. Often they are classified by the site where they start:1

  • Ductal carcinoma in which the cancer begins in the cells lining the milk ducts
  • Lobular carcinoma in which the cancer begins in the cells lining the glands [lobules] that produce milk.

Breast cancers are also classified according to the presence of certain proteins or hormones, called receptors, on the surface of the cancer cells.

Some breast cancers will be oestrogen receptor (ER) positive which means they respond and grow in the presence of the hormone oestrogen. Others will be progesterone receptor (PR) positive which means they respond and grow in the presence of the hormone progesterone. Some tumours may have no hormone receptors (i.e. ER and PR negative) and some will have both (i.e. ER and PR positive).

Some breast cancers may be classified as HER2-positive. This means that the cancer cells have a higher than normal number of HER2 receptors. HER2 is a protein that promotes cancer growth. Approximately 20% (1 in 5) of breast cancers are HER2-positive.2,3

Testing for both hormone and HER2 receptors is important because it will help your doctor to determine what the best treatment options are for you.

Who gets Breast Cancer?

Breast cancer is the most common cancer in New Zealand women, but both women and men can get breast cancer.

Around 3,000 women are diagnosed with breast cancer each year in New Zealand.4 It’s more common in older women over the age of 50,4 but can occur at any age.

There are several known risk factors for breast cancer. These include:

  • Being older
  • A previous breast cancer diagnosis
  • A family history of breast cancer
  • Having the BRCA gene
  • Obesity
  • Alcohol intake
  • Taking Hormone Replacement Therapy (treatment to help with menopause symptoms).

It’s important to remember that most women who get breast cancer will not have a family history of the disease.

These are general risk factors only and they do not necessarily mean that you will have breast cancer. Check with your doctor if you have any concerns.

  1. Slamon D et al. N Engl J Med 2011; 365: 1273-83.
  2. Piccart-Gebhart MJ et al. N Engl J Med 2005; 353: 1659-72.
  3. Salmon D et al. N Engl J Med 2001; 344, 783-92.
  4. Swain S et al. N Engl J Med 2015;372:724-34.
  5. Verma S et al. N Engl J Med 2012;367:1783-91.