Breast Cancer Causes

By Dr. Ananya Mandal, MDReviewed by April Cashin-Garbutt, MA (Editor)

The exact causes of breast cancer are not well understood. However, there are several risk factors that raise the possibilities of breast cancer. Of these risk factors some may be modified or changed like lifestyle factors while others are non-modifiable.
The causes and risk factors of breast cancer are outlined as follows –

Non-modifiable risk factors

Age

The risk of breast cancer rises with age and most cases are diagnosed in women over 50 years of age and those who have had their menopause. Eight out of 10 cases of breast cancer occur in women over 50. This means that all women between 50 and 70 years of age should be screened for breast cancer every three years. Women over the age of 70 are also likely to get breast cancer and may need regular screening.

Gender

Although women are more at risk of getting breast cancer, this cancer can also occur rarely among men. Women are 100 times more likely to get breast cancer than men.

Breast density

The normal breast contains lobules that produce milk and ducts that carry it. This glandular tissue contains a higher concentration of breast cells than other breast tissue and thus makes the breast dense. Women with more dense breast tissue may have a higher risk of developing breast cancer because there are more cells that can become cancerous. High breast density also makes diagnosis of a breast lump difficult using a mammogram. Younger women tend to have denser breasts and with age the glandular tissue in the breast decreases and is replaced by fat reducing its density.

Family history

Women who have a close relative with breast cancer or ovarian cancer have a higher risk of developing breast cancer. Although most of the breast cancers are not hereditary there are genes that determine the likelihood of getting breast cancer. This includes genes like BRCA1 and BRCA2 that can increase the risk of developing both breast and ovarian cancer. It is possible for these genes to be passed on from a parent to their child. A third gene (TP53) is also associated with increased risk of breast cancer. Other genes that have been implicated include P53, P65 and ATM. Women who have two or more close relatives from the same side of the family that includes mother, sister or daughter who have had breast cancer under the age of 50 are eligible for genetic screening to look for these genes and regular surveillance and breast cancer screening.

Those with a previous history of breast cancer

Women who have had breast cancer in one breast earlier are more likely to get breast cancer of the other breast or the same breast again.

Those with previous benign breast tumors

Usually a benign or non-cancerous lump in the breast do not indicate breast cancer. However, some of the benign breast changes may precede breast cancer. This includes atypical ductal hyperplasia (cells growing abnormally in ducts) and lobular carcinoma in situ (abnormal cells inside the breast lobes).

Exposure to estrogens

Breast cancers, especially those that are sensitive to hormones, are stimulated to grow by the female hormone estrogen. Women, who have begun their periods at a younger age and entered menopause at a late age, are exposed to longer duration of estrogen secretion from the ovaries compared to those who have had a shorter reproductive period in life. In the same way, not having children, or having children later in life, may slightly increase the risk of developing breast cancer because their exposure to estrogen is uninterrupted by pregnancy.

Height

Taller women are more likely to develop breast cancer than someone who is shorter than average. This may be due to interactions between genes, nutrition and hormones. The exact reason is unknown.

Modifiable risk factors

Being overweight or obese

Those who are overweight or obese are more at risk of developing breast cancer. It is hypothesized that these women may be having higher levels of estrogen in their blood. And women who are overweight or obese after the menopause have higher production of estrogen.

Alcohol intake

Women who take high amounts of alcohol are more likely to get breast cancer. For every 200 women who regularly have two alcoholic drinks a day, three more is diagnosed with breast cancer compared to same number of women who do not drink at all.

Exposure to radiation

Imaging studies like X rays and CT scans may raise the risk of getting breast cancer slightly. Women who have been exposed to radiation therapy for cancers earlier are at a greater risk of breast cancer.

Hormone replacement therapy (HRT)

Women who are on HRT need to take pills containing estrogen, progesterone or both. HRT is associated with slightly increased risk of developing breast cancer. Both combined HRT and oestrogen-only HRT can increase the risk of breast cancer, although the risk is slightly higher with combined HRT. There are around 19 extra cases of breast cancer for every 1,000 women taking combined HRT for 10 years. The risk reduces after stopping HRT.

Exposure to Diethylstilbesterol (DES) 

Women who took diethylstilbestrol (DES) to prevent miscarriage may have an increased risk of breast cancer after age 40. This drug was given to the women in the 1940s - 1960s.

Breast cancer and breast implants

Risk of breast cancer is not raised by breast implants (those approved by regulatory authorities), using antiperspirants, exposure to pesticides and wearing underwired bras.

Further Reading

• All Breast Cancer Content
• What is Breast Cancer?
• Breast Cancer Classification
• Breast Cancer Symptoms
• Breast Cancer Pathophysiology