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Causes of breast cancer

Updated on May 16, 2008

The exact cause or causes of breast cancer is not clear. However, some risk factors have been identified that increase a person's chance of getting this disease. Generally these factors are divided into two groups, factors that can not be modified, factors that can be modified. Research has shown that women with certain risk factors are more likely than others to develop breast cancer. A risk factor is something that may increase the chance of developing a disease.


The risk of breast cancer increases with aging. For example, annual breast cancer rates are 8-fold higher in women who are 50 years old, in comparison with women who are 30. Most breast cancers (about 80%) develop in women over the age of 50. In one age group (40 to 45 years), breast cancer is ranked first among all causes of death in women. Breast cancer is uncommon in women younger than 35, with the exception of those who have a family history of the disease.

Previous Breast Cancer

If a woman has already had breast cancer, she has a greater chance of developing a new cancer in the other breast. Such a new, or 'second,' cancer arises from a completely different location and should not be confused with a cancer that has recurred (come back) or metastasized (spread) from another site. The likelihood of a new cancer increases by 0.5% to 0.7% each year after the original diagnosis. After 20 years, a woman has a 10% to 15% chance of developing a new breast cancer. A previous diagnosis of lobular carcinoma in situ (a localized tumor) is associated with a 10% to 30% greater breast cancer risk, and a previous diagnosis of ductal carcinoma in situ is associated with a 30% to 50% greater risk.

Breast Disease

complex fibroadenoma (fibrous, benign tumor of glandular tissue), hyperplasia (abnormal increase in cell number), or atypia (abnormal cellular structure) are breast diseases. Moderate or severe hyperplasia alone may increase breast cancer risk by 1.5- to 4-fold; however, when associated with atypia, the risk may be increased as much as 5-fold. If a woman also has a family history of breast cancer in first-degree relatives, her risk may be increased 11-fold.

Family History of Breast Cancer

Approximately 85% of women with breast cancer do NOT report a history of breast cancer within their families. Of the remaining 15%, about one-third appears to have a genetic abnormality. Women who have a first-degree relative (mother, sister, or daughter) with breast cancer have almost doubled breast cancer risk. The risk is increased 4- to 5-fold if the relative's cancer was found before menopause (the end of menstruation) and involved both breasts. The risk also is increased if breast cancer occurs in several family generations. A woman's risk of breast cancer is higher if her family member got breast cancer before age 40. Having other relatives with breast cancer (in either her mother's or father's family) may also increase a woman's risk.

Genetic Mutations

Genetic mutations (changes in genetic information carried on DNA or RNA) are primary cause for most cancers. Other risk factors actually cause cancers indirectly through altering genes. Some genes contain instructions for controlling when our cells grow, divide, and die. Oncogenes are genes that speed up cell division. Tumor suppressor genes are those that slow down cell division, or cause cells to die at the right time. Normally, these genes are well balanced in their activities. Genes, BRCA1 and BRCA2, have been most studied. These genes are tumor suppressor genes. Mutations in these genes can be inherited from one's parents. When they are mutated, they no longer function to suppress abnormal growth, and cancer is more likely to develop. Some descendants of Ashkenazi Jews from Eastern and Central Europe have an inherited BRCA1 mutation. Each will have up to a 90% lifetime risk of developing breast cancer. More than half will be diagnosed with breast cancer by age 50. The BRCA2 genetic mutation also is prevalent among families with Ashkenazi backgrounds. In addition, many other genes, including the genes named p53, AT, the GADD repair group, the RB suppressor gene, and the HER-2/neu oncogene (a gene that contributes to cancer) may be associated with breast cancer. Some of them directly influence breast cancer risk, others are involved in the general processes of cancer growth and metastasis.


Breast cancer risk is increased in women with the longest known exposures to sex hormones, particularly estrogen (female sex hormone). Therefore, breast cancer risk is increased in women who have a history of early first menstrual period (before age 12), late menopause (end of menstruation after 55), no pregnancies, late pregnancy (after age 30), or use of birth control pills. Estrogen replacement therapy (ERT used by many older women to relieve the symptoms of menopause may increase the risk of breast cancer after long-term use (10+ years).

Body fat

Although a woman's ovaries stop making sex hormones after menopause, her body still produces estrogen. Aromatase, an enzyme synthesized by body fat, participate in estrogen production from androstenedione, a steroid released by the adrenal glands. Postmenopausal women with more body fat have more aromatase. Therefore, they can convert more androstenedione into estrogen. Therefore, body fat may increase breast cancer risk indirectly by increasing estrogen levels in the body.

Alcohol Use

The risk of breast cancer is increased among women who drink. Breast cancer risk is nearly doubled in women who have more than three drinks daily. Alcohol consumption may increase cancer risk indirectly through increased level of estrogen in the blood.

Radiation Exposure

A significantly increased risk of breast cancer has been found in women who received radiation therapy in the chest area during childhood or young adulthood. Women treated with radiation for Hodgkin's lymphoma have higher cancer risk. Studies show that the younger a woman was when receiving radiation treatment, the higher her risk of breast cancer later in life. Exact causes of breast cancer have not been clearly identified yet. Many risk factors have been discussed which may cause breast cancer. Minimizing our exposure to these known risk factors can effectively protect us against breast cancer.


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    • tinyteddy profile image

      tinyteddy 9 years ago from INDIA

      good attempt but lacks insight

    • susanzheng profile image

      susanzheng 9 years ago

      Hello, fellow hubers, please read my new hub which may help you, your friends or family members to take some preventive measures for breast cancer. I like to have your feedback and suggestions about how to improve the information and writing.

      Thanks and have a wonderful day.