Breast Cancer Risk Factors

Breast cancer is the most common cancer affecting women, yet no two women have the same level of risk. A combination of genetic, hormonal, reproductive, lifestyle, and environmental factors influences an individual's likelihood of developing breast cancer, with some factors being beyond our control and others potentially modifiable. As a specialist oncoplastic and reconstructive breast surgeon in Melbourne, one of my key goals is to help women understand their personal level of risk rather than relying on a "one-size-fits-all" approach to screening and prevention. Understanding your risk factors allows for more informed decisions about breast cancer screening, risk reduction strategies, and, where appropriate, genetic testing.

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Breast Cancer Risk Factors That You Cannot Control

The most important breast cancer risk factors are female sex and age. Women are 100 times more likely to get breast cancer than men. The risk increases with advancing age: 75 per cent of breast cancers occur after the age of 50.

Genetics play an important role in only 5-10 per cent of breast cancers. In other words, 90 per cent of the breast cancers are so called sporadic (by chance). Based on the number of affected first and second-degree relatives with breast or ovarian cancer, women are divided into three categories: 

• Average risk: 1 in 11 to 1 in 8 chance of breast cancer by the age of 75. 

•  Moderate risk: 1 in 8 to 1 in 4 chance of breast cancer by age of 75. 

•  High risk:  More than 1 in 4 chance of breast cancer by age of 75.

If you have multiple family members with history of breast cancer specially at young age, or any relative with history of ovarian cancer, or breast cancer in a male relative it is important to see a breast specialist to go through your family history and determine your risk. This has practical implications to determine the intensity and modality of breast cancer screening in women as well as approaches to manage their risk. For example, screening for women at average risk of breast cancer is done by two yearly mammography through BreastScreen. If you are a woman who is at high risk of familial breast cancer, further assessment in a familial breast cancer clinic is warranted. This includes genetic assessment, annual screening often with breast MRI from as young as 25 years of age, and discussions about risk reducing surgery and medications.

Breast Cancer Risk Factors That You Can Control

There are some ‘so called’ modifiable breast cancer risk factors such as obesity, alcohol use, high fat diet, and sedentary lifestyle. Although each one in isolation is considered a minor risk factor, presence of multiple of these risk factors together can add up to a substantially increased risk of breast cancer. Prolonged combined hormone replacement therapy (HRT) in postmenopausal women has been shown to increase risk of future hormone positive breast cancer and therefore risks and benefits of HRT should be considered on an individual basis. In women with high risk of familial breast cancer risk-reducing interventions may be discussed. The most effective risk-reducing strategy is bilateral mastectomy which reduces future risk of breast cancer by 90-95%. The other option is taking tamoxifen which reduces the risk by about 50%.

How Is Your Breast Cancer Risk Calculated?

While individual risk factors such as age, family history, breast density, reproductive history, and lifestyle all influence breast cancer risk, they do not act in isolation. Instead, these factors interact in complex ways, making it difficult to accurately estimate risk based on any single characteristic alone.

To overcome this, validated breast cancer risk prediction models have been developed to combine multiple risk factors into a single personalised estimate of risk. One of the most widely used and extensively validated models is the Tyrer–Cuzick (IBIS) Risk Model, which incorporates information such as:

• Age

• Family history of breast and ovarian cancer

• Known genetic mutations (such as BRCA1 or BRCA2)

• Breast density

• Previous breast biopsies and benign breast disease

• Reproductive and hormonal factors

• Body mass index (BMI)

• Personal history of breast cancer

Using this information, the model estimates both your 10-year risk and lifetime risk of developing breast cancer. These estimates help determine whether you are at average, moderate, or high risk and can guide recommendations regarding the most appropriate screening strategy, including when to start screening, how often it should be performed, and whether additional imaging such as breast MRI or contrast-enhanced mammography may be beneficial.

It is important to remember that a risk estimate is not a diagnosis. A woman with a higher estimated risk may never develop breast cancer, while someone at average risk can still be diagnosed with the disease. The purpose of risk assessment is to identify women who may benefit from more personalised screening and earlier detection.