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HomeYour HealthConditionsCancerBreast CancerWhat Are The Causes And Risk Factors for Breast CancerWhat Are The Causes And Risk Factors for Breast Cancer?

Published on Dec 22, 2023

Authored by Pfizer Medical Team

Breast cancer is the most commonly diagnosed cancer in females in Australia1, and the third most common cancer in New Zealand2. Although many women are affected by breast cancer, people may have questions about it. Read on to learn the facts.

What Is Breast Cancer?

Breast cancer is caused by the uncontrolled growth of cells in the breast. These cells can form a mass called a tumour. Although breast cancers can start in different parts of the breast, most begin in the glands that produce milk (called lobules) or in the tubes (called ducts) that bring milk to the nipple (see Figure 1).

Figure. 1. Diagram of the female breast

What Is Breast Cancer? What are the Types of Breast Cancer?

There are many different types of breast cancer. The most common types are carcinomas, which are named based on where they form and how far they have spread. The most common types of breast cancer are called ductal carcinoma in situ, invasive ductal carcinoma, and invasive lobular carcinoma.

Invasive means that the cancer has spread to other tissue in the breast.

In situ is a non-invasive condition in which abnormal cells are found in the tissue where they began growing and have not invaded other tissue layers. 

There are two types of in situ cancers:3

  • Ductal carcinoma in situ (DCIS) also called intraductal carcinoma - a non-invasive breast cancer, or breast cancer where the abnormal cells have not spread outside the duct to other tissues in the breast. In some cases, ductal carcinoma in situ may become invasive cancer and spread to other tissues. There is currently no way to know which lesions could become invasive.
  • Lobular carcinoma in situ (LCIS) - an uncommon change in the breast that isn’t cancer. Abnormal cells that look like cancer are present within, but not outside of the lobules in the breast.

There are many different kinds of invasive breast cancer, but the two most common are:3

  • Invasive ductal carcinoma - when the cancer cells spread from the ducts to other parts of the breast tissue. They can also spread (metastasise) to other parts of the body.
  • Invasive lobular carcinoma - when the cancer cells spread from the lobules to other parts of the breast tissue that are nearby. They can also spread (metastasise) to other parts of the body.

Less common types of breast cancer include inflammatory breast cancer, Paget disease of the nipple, Phyllodes tumour and angiosarcoma.3

While most breast cancers occur in women, men can also develop breast cancer.

What Causes Breast Cancer?

A breast cancer risk factor is anything that makes it more likely you'll get breast cancer. Having one or more risk factors does not mean that a woman will develop breast cancer. However, there are certain risk factors that can increase a woman’s risk of developing breast cancer, including:4

  • Age: Most breast cancers are found in women over 50 years
  • Certain genetic mutations: Women who inherit mutations (changes to certain genes - for example, BRCA1 and BRCA2, pronounced ‘bracka-one’ and ‘bracka-two’) are at higher risk
  • Getting periods before age 12 or starting menopause after age 55: This causes a woman to be exposed to certain hormones for a longer period of time, which may slightly increases her risk for breast cancer
  • Having dense breasts: This can make it harder to see tumours on a mammogram. Dense breasts have more connective tissue than fatty tissue
  • Family history: Having a first-degree relative (mother, sister or daughter) or more than one family member on either the mother’s or father’s side who have had breast cancer increase a woman’s risk
  • Lifestyle factors: Smoking, being overweight or obese, and regular alcohol consumption may increase the risk of breast cancer

Research has shown that most breast cancers are caused by damage to genes that happens by chance after a person is born.5 Only 5-10% of breast cancers are inherited (when gene changes - called mutations - are passed from one generation of a family to the next).4,5

Do Genetic Mutations Cause Breast Cancer?


​​Women who inherit genetic mutations to certain genes (such as BRCA1 ‘bracka-one’ and BRCA2 ‘bracka-two’) have a higher risk of developing breast cancer.6

While all women have BRCA1 and BRCA2 genes, mutations to these are rare. Not every woman with a mutation will get breast cancer, but it does put her at increased risk. Breast cancers linked to the BRCA1 or BRCA2 mutations are often found in younger women and more often in both breasts. Mutations in the BRCA1 and BRCA2 genes also increase a woman’s risk of developing ovarian cancer.6

In addition to mutations in the BRCA1 and BRCA2 genes, there are a number of other, less-common genetic mutations that can also increase a woman’s risk of developing breast cancer - though not as much as BRCA1 and BRCA2 mutations. These include ATM, TP53, CHEK2, PTEN, CDH1, STK11 and PALB2 genes.7

Genetic Testing for Breast Cancer

Genetic testing can identify some women with inherited mutations in their BRCA1 and BRCA2 genes, as well as in certain other genes and biological markers. A biological marker, or biomarker, is a useful measurement that helps your doctor understand your health and make important decisions about your care. More details about biomarkers can be found here.

This information can help women work with their cancer specialist, known as an oncologist, to:

  • Reduce their risk of developing breast cancer
  • Start screening for breast cancer earlier than a person who does not have the mutations
  • Take steps to look for and possibly prevent other cancers that are influenced by BRCA1 and BRAC2 genes
  • Learn if certain treatments are likely to be effective
  1. Australian Institute of Health and Welfare. BreastScreen Australia monitoring report 2023, Catalogue number CAN 155. Accessed Nov 2023.

  2. Ministry of Health New Zealand. Breast Cancer. Accessed November 2023.

  3. Breast Cancer Network Australia. Types of Breast Cancer. Accessed Nov 2023.

  4. Breast Cancer Network Australia. Risk Factors. Accessed Nov 2023.

  5. Cancer.Net. Breast Cancer: Risk Factors and Prevention. Accessed Dec 2023.

  6. Kuchenbaecker KB, Hopper JL, Barnes DR, et al. Risks of Breast, Ovarian, and Contralateral Breast Cancer for BRCA1 and BRCA2 Mutation Carriers. JAMA. 2017;317(23):2402-2416.

  7. American Cancer Society. Breast Cancer Risk Factors You Cannot Change. Accessed Nov 2023.

External Resources

Breast Cancer Foundation NZ
Cancer Council
Peter MacCallum Cancer Centre: iPrevent
Healthdirect: Breast Cancer
McGrath Foundation: Find a McGrath Breast Care Nurse

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