What Causes Breast Cancer?

Breast cancer is cancer that affects the breast. The vast majority of breast cancer occurs in women, although men can develop breast cancer too. ‘Cancer’ is the name for a group of diseases in which the body’s cells are changed in appearance and function. Such abnormal cells can grow out of control and form a mass or ‘tumor.’ When abnormal cells originate in the breast tissue, the mass is called a breast tumor.

Causes of Breast Cancer

The exact cause or causes of breast cancer remain unknown. Yet scientists have identified a number of risk factors that increase a person’s chance of getting this disease. Certain risk factors, such as age, are beyond our control; whereas others, like drinking habits, can be modified.


The risk of breast cancer increases with age. 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
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.

Past family history

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. The risk of breast cancer is about two times higher among women who have a first-degree relative (mother, sister, or daughter) with this disease. The risk is increased 4-5-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.

Genetic causes

In addition, an increased risk of breast cancer has been found in families with other inherited disorders, such as ataxia telangiectasia (a progressive disease of the motor system) and Li-Fraumeni syndrome.

Hormones causes

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
  • No pregnancies,
  • Late pregnancy (after age 30)
  • Birth control pills

It should be mentioned that the Pill’s exact hazards are difficult to assess, since risk apparently disappears in women who have not used oral contraceptives for more than 10 years.

Additional Reading on Breast Cancer

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