What causes breast cancer
Last updated: April 1, 2026
Key Facts
- Age is the most significant risk factor; approximately 80% of breast cancers occur in women over 50 years old
- BRCA1 and BRCA2 gene mutations increase lifetime breast cancer risk to 45-87%, with inherited mutations accounting for 5-10% of cases
- Estrogen exposure from hormone replacement therapy (HRT), late menopause, and early puberty increases breast cancer risk significantly
- Obesity and excess body fat increase estrogen production through aromatase enzyme activity in adipose tissue
- Alcohol consumption increases estrogen levels and decreases folate absorption, with two drinks daily raising risk by approximately 20%
Breast Cancer Risk Factors
Breast cancer is the most common cancer in women, with multiple interconnected risk factors contributing to development. Understanding these factors is crucial for risk assessment and prevention strategies. Most women diagnosed with breast cancer have no family history, indicating that environmental and lifestyle factors play significant roles alongside genetics.
Genetic and Hereditary Factors
While most breast cancers are not hereditary, genetic mutations dramatically increase risk. BRCA1 and BRCA2 mutations are responsible for most hereditary breast cancer cases. Women with these mutations have a 45-87% lifetime risk of developing breast cancer compared to 13% in the general population. Other genes like TP53, PTEN, and CDH1 also increase susceptibility. Genetic testing is recommended for women with strong family histories or early-onset breast cancer.
Hormonal Risk Factors
Estrogen and progesterone exposure significantly influence breast cancer development. Prolonged menstrual history increases cumulative hormone exposure, with early menarche (before age 12) and late menopause (after age 55) increasing risk. Hormone replacement therapy (HRT) during menopause increases breast cancer risk, particularly for combined estrogen-progestin formulations. Oral contraceptives also slightly increase risk while in use, with risk returning to baseline after discontinuation.
Obesity and Metabolic Factors
Post-menopausal obesity significantly increases breast cancer risk through multiple mechanisms. Excess adipose tissue produces estrogen through aromatase enzyme activity, elevating circulating hormone levels. Obesity also increases insulin-like growth factor (IGF-1), promotes inflammation, and impairs immune surveillance. Interestingly, pre-menopausal obesity shows less association with breast cancer risk, possibly because ovarian estrogen production dominates over adipose tissue production.
Lifestyle and Environmental Factors
Alcohol consumption increases breast cancer risk by approximately 20% with two drinks daily, with higher consumption increasing risk proportionally. Mechanisms include increased estrogen metabolism, reduced folate absorption, and liver injury. Smoking shows inconsistent associations but may increase risk in specific subgroups. Physical inactivity contributes through obesity and metabolic dysfunction. Exposure to ionizing radiation, particularly during adolescence, increases risk significantly.
Reproductive Factors
Pregnancy and breastfeeding reduce lifetime breast cancer risk. Women who never have children have higher risk than those with multiple pregnancies. Breastfeeding provides protective benefits in a dose-dependent manner. Later age at first pregnancy increases risk compared to earlier pregnancies. These factors reflect the protective effect of lactation-induced cell differentiation in breast tissue.
Age and Dense Breast Tissue
Increasing age is the strongest non-modifiable risk factor for breast cancer. Mammographically dense breast tissue increases both cancer risk and detection difficulty. Women with extremely dense breasts have 4-5 times higher risk. Dense tissue contains more epithelial cells susceptible to malignant transformation compared to fatty tissue.
Related Questions
Can men get breast cancer?
Yes, though rare, accounting for approximately 1% of all breast cancers. Men diagnosed with breast cancer typically have worse outcomes due to later detection stages. BRCA2 mutations and family history increase male breast cancer risk.
How is breast cancer diagnosed?
Breast cancer is typically diagnosed through mammography, ultrasound, or MRI imaging, followed by biopsy for tissue confirmation. Clinical examination and patient-reported symptoms also prompt diagnostic testing.
What are the survival rates for breast cancer?
Five-year survival rates exceed 90% overall, with higher rates for early-stage cancers (99% for stage 1). Survival varies significantly based on cancer type, stage, grade, and patient factors including age and overall health.
Sources
- National Cancer Institute - Breast Cancer Risk Factors Public Domain
- Wikipedia - Breast Cancer CC-BY-SA-4.0
- Mayo Clinic - Breast Cancer CC-BY