The cause of many breast cancer patients is not clear. In fact, nearly half of breast cancer patients do not have a clear risk factor.
Risk factors are factors that make an individual more likely to develop the disease, and risk factors vary from cancer to cancer. However, the presence of one or more cancer risk factors does not necessarily mean that one will develop cancer. Many women with one or more risk factors for breast cancer do not develop breast cancer, and about half of those with breast cancer do not have significant risk factors.
Factors that significantly increase the risk of breast cancer
- Medical history. Women who have had cancer in one breast (such as ductal carcinoma in situ or invasive breast cancer) are 3 to 4 times more likely to have a new breast cancer in the opposite breast or in a different part of the same breast that is not related to the current breast cancer. This is different from breast cancer recurrence.
- Age. Breast cancer risk increases with age. About 77% of women diagnosed with breast cancer each year are older than 50 years old, and nearly 50% are over 65 years old.
The risk of breast cancer is 1 in 68 for women 40 to 50 years old; increases to 1 in 42 for women 50 to 60 years old; 1 in 28 for women 60 to 70 years old; and 1 in 26 for women over 70 years old.
Factors that moderately increase the risk of breast cancer
- Breast cancer in an immediate family member. Breast cancer in a mother, sister, or daughter (first-degree relative) increases a woman’s risk of developing cancer. The risk is higher if a first-degree relative had breast cancer before menopause and has cancer in both breasts. 1 first-degree relative with breast cancer approximately doubles a woman’s risk, and 2 first-degree relatives with breast cancer triple the risk. Breast cancer in a male relative also increases a woman’s risk of developing the disease.
- Gene. About 5% to 10% of breast cancers are hereditary. People who carry familial mutations in the breast cancer genes BRCA1 and BRCA2 have an increased risk of developing cancer. Women who carry a mutation in the BRCA1 gene have a 55% to 65% lifetime chance of developing breast cancer, and women who carry a mutation in the BRCA2 gene have a 45% lifetime chance of developing breast cancer.
- Breast lesions. Atypical hyperplasia or lobular carcinoma in situ from a previous breast biopsy increases a woman’s risk of breast cancer by 4 to 5 times.
Factors that slightly increase the risk of breast cancer
- Distant relative with breast cancer. Refers to second- or third-degree relatives who have breast cancer, such as aunts, grandmothers, and cousins.
- Abnormal previous breast biopsy. Women with a previous biopsy showing a slightly increased risk of breast cancer for any of the following: complex fibroadenoma, mastopexy without atypical hyperplasia, sclerosing adenopathy, and solitary papilloma.
- Older age at childbirth. Birth of the first child after age 35 or never having had a child increases the risk of breast cancer.
- Early menarche. Prolonged exposure to endogenous (self) estrogen increases breast cancer risk, such as menarche before age 12, menopause after age 55, and never having been pregnant.
- Weight. Being overweight (especially with a large waist circumference) and having excess calorie and fat intake increases risk, especially if you are overweight after menopause.
- Exposure to excessive radiation. Women who receive too much radiation before age 30 are especially at increased risk, often due to radiation received for cancer treatment.
- Family history of other cancers. If a family member had ovarian cancer before age 50, there is an increased risk of developing breast cancer yourself.
- Alcohol. Drinking alcohol increases the risk of breast cancer. Women who drink 1 drink a day have a slightly increased risk of breast cancer compared to non-drinkers, and women who drink 2 to 5 drinks a day have about 1.5 times the risk of developing cancer.
- Hormone replacement therapy (HRT). Long-term use of combined estrogen and progestin therapy increases the risk of breast cancer. This risk appears to regress to the general population after interruptions of treatment for ≥5 years.
Factors that reduce breast cancer risk
- Less lifetime exposure to endogenous estrogens. Pregnancy before age 18, early menopause, and removal of the ovaries before age 37 reduce the risk of breast cancer.
- Lactation. Women who breastfeed have a reduced risk of breast cancer.
Factors that do not affect breast cancer risk
- Fibrocystic changes in the breast.
- Multiple pregnancies.
- Coffee or caffeine intake.
- Use of antiperspirants.
- Wearing a bra with a steel bra.
- Use hair dyes.
- Miscarriage.
- Implantation of breast implants.
Researchers are still studying whether smoking, high-fat diets, lack of exercise, and environmental pollution increase breast cancer risk. Some studies have shown a slight increase in the risk of breast cancer in women who take birth control pills. The risk disappears when you stop taking it for 10 years or more, but other studies have not shown a correlation, and related research is still ongoing.