The causes of breast cancer may be sporadic, familial, or hereditary. Sporadic breast cancer means breast cancer that is not due to an inherited gene mutation. There are multiple risk factors responsible for sporadic breast cancer: hormonal, dietary, lifestyle-induced, benign breast diseases, and environmental factors are significant causes of breast cancer.
A majority of breast cancer cases are sporadic (approximately 70%), while 20-25% causes of breast cancer are familial, and only 5-10% are hereditary. Hereditary causes of breast cancer involve mutated genes like BRCA. Familial causes of breast cancer do not involve any apparent mutations in genes, but other family members are at higher risk of developing breast or other cancers. Some causes can be modified, and their presence indicates that regular screening or active surveillance is necessary.
Non-modifiable risk factors: Genetic causes of breast cancer
Being a woman is the most significant risk factor for breast cancer. Women have a 100 times higher risk than men do. Male breast cancer contributes less than 1% of the total breast cancer burden. Here are some other risk factors for breast cancer:
Age
Women aged above 50 are at more risk of developing breast cancer than younger women. Women in their 70s have the highest risk. In the last decade, because of the change in lifestyle and environment, the number of young breast cancers is increasing.
Family history
Breast cancer (and the genetic mutations responsible for the disease) can be passed on within a family. Having a close blood relative with confirmed breast cancer increases the risk of developing the disease, especially in later years. A woman is considered to have twice the risk of developing breast cancer, if a direct female blood relative such as her mother, sister (or even daughter) is diagnosed positive. If a woman has two or more first-degree relatives with breast cancer, her risk of developing the disease increases threefold. Up to 10% of all cases of breast cancer are hereditary.
Personal history of having breast cancer
Patients with a prior history of breast cancer are at a higher risk of developing contralateral breast cancer (in the opposite breast), or in a different quadrant of the same breast, if the patient has undergone breast conservative surgery.
Genetic abnormalities
Hereditary breast cancer constitutes 5-10% of all breast cancers. The most common genetic mutations occur in the BRCA1 and BRCA2 genes. Women with a mutated BRCA1 gene have a 60-85% lifetime risk of developing breast cancer while BRCA2 mutated patients have a 40-60% lifetime risk. BRCA mutated patients also have a 20-30% lifetime risk of ovarian cancer. There are other genes like p53, PTEN, ATM, which can lead to hereditary breast cancer.
Race and ethnicity
There is a direct link between breast cancer and race. For example, white Caucasian women are at a higher risk of developing breast cancer when compared to Asian, African-American, or Hispanic women. Survival outcomes also vary with race. For example, the probability of breast cancer-related death is higher in African American women and Hispanic women than in Caucasian women. Also, African-American women are most likely to develop an aggressive form of breast cancer at a much younger age than women of other races and ethnicities.
History of menstrual activity
Early menarche and late menopause are important risk factors for breast cancer. This means that women who attain puberty before the age of 12 (with early menstrual activity), and women who attain menopause after the age of 55 are at a higher risk of developing breast cancer in their lifetime. Prolonged exposure towards estrogen stimulates the growth of cells within breast tissues. Similarly, exposure to hormone replacement therapies (HRT) after menopause can also increase the risk of breast cancer.
Dense breasts
Breast density is a significant risk factor. Patients with high breast density are at 2-4 times higher risk of developing breast cancer. Having dense breasts also makes it harder for mammograms to detect a cancerous lesion in the breast tissues at the early stage.
Previous breast changes/breast abnormalities
A woman is considered to be at higher risk of developing breast cancer, if they have a personal history of:
- benign breast lumps
- Lumps in the axilla
- breast cancer
Modifiable risk factors: Environmental and lifestyle causes of breast cancer
Lack of physical activity
People with low or no physical activity are at an increased risk of developing breast cancer. Research has shown that 4 to 7 hours of moderate-to-intense exercise every week can significantly reduce the risk of breast cancer.
Being overweight/obese
Women who continue to remain overweight/obese after menopause are more likely to develop breast cancer. This risk is commonly attributed to the secretion of estrogen by fat cells, after the age of menopause. Also, being overweight increases the levels of insulin present in the blood, which in turn boosts a woman’s breast cancer risk. Women who have completed breast cancer treatment are also at a higher risk of a relapse, if they continue to remain overweight.
Alcohol consumption
Studies have shown that women who consume two or more alcoholic beverages in a day are at a 1.5 times higher risk of developing breast cancer as compared to women who do not consume alcohol. This risk is seen to increase with more significant proportions of daily alcohol intake.
Smoking
Young and pre-menopausal women who smoke are at a higher risk of developing breast cancer. Research has also demonstrated the links between massive second-hand smoke exposure and breast cancer risk in postmenopausal women.
Exposure to radiation (before the age of 30)
Women exposed to prolonged chest radiation at an early age are at an increased risk of developing breast cancer later in their lives. Women who have received radiation to the chest to treat any type of cancer other than breast cancer (such as Hodgkin’s lymphoma, non-Hodgkin’s lymphoma, chest wall sarcoma) are also at a higher risk of developing breast cancer than those who have not received radiation therapy.
Pregnancy timelines
Women who do not get pregnant at all (nulliparous), or who get pregnant after the age of 30.
Breastfeeding history
Women who have breastfed their babies for one year or more are at a lower risk of developing breast cancer as compared to those who have not.
History of Hormone Replacement Therapy (HRT)
Hormone Replacement Therapy (HRT) refers to the treatment used to relieve women from the symptoms of menopause. HRT replaces hormones that are at a lower level when women approach menopause. Women who are currently receiving HRT or have received it in the near past, have a higher risk of developing breast cancer.
Risk factors with unclear effect factors of breast cancer
Deficiency of vitamin D
Women with low levels of vitamin D are at an increased risk of developing aggressive/invasive forms of breast cancer. Vitamin D plays a vital role in controlling the regular growth rate of breast cells and may be able to stop breast cancer cells from growing.
Exposure to artificial light at night
Survey results have shown that women who work in night shifts, including doctors, nurses, and police officers, are at a higher risk of developing breast cancer compared to women who work during daylight. Increased exposure to artificial light at night causes this. Women who live in areas with high levels of artificial external light during the night (e.g., street light) also face a higher breast cancer risk.
Eating junk/unhealthy food
Unhealthy dietary habits are partly responsible for breast cancer. Whole foods and foods rich in antioxidants can boost human immune activity, and reduce the risk of breast cancer.
Eating processed foods or chemically preserved foods
Processed foods may include pesticides, antibiotics, and hormones used on both crops and livestock. Research shows that these can contribute to an increase in the risk for breast cancer. The risk is also increased by mercury found in seafood and industrial chemicals used in the food and food packaging industry.
Eating grilled meat
Research shows that women who eat a lot of grilled, barbecued, or smoked meats are at a higher risk of breast cancer when compared to those who include more of fruits and vegetables in their diet.
Heavy use of cosmetics
Recent research suggests that exposure to certain chemicals found in cosmetics can contribute to the growth of breast tumours.
Excessive use of sunscreen
The use of sunscreen is intended to protect us against UV rays. However, recent research suggests that excessive levels of exposure to some of the chemicals that are found in sunscreens can lead to breast cancer.
High exposure to plastic
Research shows that at certain exposure levels, some of the chemicals used in the manufacture of plastic products, such as bisphenol A (BPA), may act as carcinogens and increase the risk of breast cancer.
Exposure to chemicals in gardening products
Research-based evidence supports the theory that chemicals used in lawn and garden products may increase cancer risk. However, it has been difficult to establish definitive cause and effect for any specific chemical.
Drinking contaminated water
More than 206 contaminants found in drinking water have been identified to date. There is no conclusive evidence relating these pollutants directly with breast cancer. Government bodies are regulating some of these contaminants (such as chlorine and fluoride).