Breast Cancer Causes & Risk Factors

There are several risk factors that have been identified as characteristics that can potentially increase a person’s likelihood of developing breast cancer. Risk factors do not necessarily cause breast cancer, however. Having one or more risk factors does not guarantee a person will develop the condition, and some people with identified risk factors never develop breast cancer. In addition, some risk factors are controllable, while others are not.

In general, the National Cancer Institute estimates that the average American woman has approximately a 1 in 8 chance of developing breast cancer in her lifetime.¹

Gender

Women are much more likely to develop breast cancer than men, although breast cancer can and does develop in men. In 2018, an estimated 266,120 women and 2,550 men were predicted to be diagnosed with breast cancer.² In men, risk factors for breast cancer include obesity, Klinefelter syndrome (a rare genetic syndrome), and having excess breast tissue (gynecomastia).³

Age

The biggest risk factor for breast cancer is age: a woman’s risk of developing breast cancer increases as she ages.¹ Most breast cancers occur in women over the age of 55,⁴ although breast cancer can occur in younger women as well.

Genetic mutations

Certain genetic mutations have been identified that increase a person’s risk of developing breast cancer, including BRCA1 and BRCA2. These inherited genetic mutations are believed to account for no more than approximately 10% of all breast cancers, but women who have these mutations are at an increased risk of developing the disease.¹

Other genetic mutations that are less common but may also slightly increase a person’s risk of breast cancer include ATM, TP53, CHEK2, PTEN, CDH1, STK11, and PALB2.⁴

Family history

An individual who has a first-degree relative (a mother, sister, or daughter) who has had breast cancer is more likely to develop the disease. Women who have a close male relative who has had breast cancer are also at an increased risk of developing breast cancer.¹

Breast density

Breast density is measured by mammograms. Glandular (the milk glands) and connective tissue appear white on mammograms, while fatty tissue in the breast appears dark. Women who have a higher percentage of dense tissue as seen on a mammogram are at an increased risk of developing breast cancer.¹

Personal history of breast cancer

If a woman has had breast cancer before, she is at an increased risk of developing a second breast cancer.¹

Certain changes in breast tissue

Certain changes in breast tissue that may be discovered during a biopsy can increase a person’s risk of developing breast cancer. These include:¹

• Atypical hyperplasia – a condition in which cells are abnormal and increased in number, but not cancerous
• Lobular carcinoma in situ (LCIS) – a condition in which abnormal cells are found in the lobules (milk glands)
• Ductal carcinoma in situ (DCIS) – a condition in which abnormal cells are found in the ducts

Generally, women with atypical hyperplasia or LCIS are monitored carefully but not given active treatment. Because DCIS can develop into invasive cancer, women with DCIS are usually given treatment, such as surgery and/or radiation.¹

Previous radiation therapy

If a person has had previous radiation therapy to the chest before age 30, such as for the treatment for Hodgkin lymphoma, it can increase their risk of developing breast cancer. The younger the person was when they received the radiation therapy, the greater the risk for breast cancer.¹

Alcohol use

Research has suggested that the more a woman consumes alcohol, the greater the risk of developing breast cancer.¹

Reproductive and menstrual history

A woman’s reproductive and menstrual history can also impact her risk for breast cancer. Women who began menstruating before age 12 or who went through menopause after age 55 have an increased risk for breast cancer. In addition, women who have never had a full-term pregnancy or who had their first full-term pregnancy after age 30 are at an increased risk of developing breast cancer.¹

Long-term use of hormone therapy for menopause

Long-term use (more than 5 years) of combined estrogen and progestin hormone therapy for menopause increases a woman’s risk of developing breast cancer.¹

Previous use of the drug DES (diethylstilbestrol)

Between 1940 and 1971, some pregnant women received the drug DES to prevent miscarriage. It is now known that women who took this drug during pregnancy have a somewhat increased risk of developing breast cancer. Women whose mothers received DES during pregnancy may have a somewhat increased risk of developing breast cancer after the age of 40.¹

Increased weight in post-menopausal women

Post-menopausal women who have not used menopausal hormone therapy and who are overweight or obese have an increased risk of developing breast cancer compared to women of a healthy weight.¹

Low physical activity levels

Physical activity has many positive health benefits, and women who are physically inactive throughout their life may be at an increased risk of developing breast cancer.¹

Ethnicity

Breast cancer is slightly more common in white women than in other ethnic groups, although women of all ethnicities can develop breast cancer.¹