Breast cancer: causes and risk factors

The exact causes of breast cancer aren't known. Nonetheless, there are certain circumstances that have been shown to increase a person’s vulnerability to cancer.

The following risk factors have been identified:

- Female gender: about 11-12% of women develop breast cancer.

- More advanced age: 77% of women diagnosed are over 50.

- Having one first-degree relative (mother, sister or daughter) with breast cancer doubles risk; having two increases risk five-fold.

- Approximately 5-10% of breast cancers are because of inherited mutations (changes) of the genes BRCA1 and BRCA2, which normally make cancer-protective proteins. This function is less effective with a mutated gene; 50-60% of women with BRCA1 or BRCA2 mutations develop breast cancer by age 70.

- Risk increases significantly if you’ve had cancer of the breast, ovaries, uterus or colon. Cancer in one breast carries a 1% chance per year of developing cancer in the other breast.

- When a biopsy indicates abnormal cells that aren’t yet cancerous (atypical hyperplasia), there is moderately increased risk of developing cancer in future. Atypical hyperplasia is a benign condition associated with abnormal cell growth. Young patients with this diagnosis are very likely to develop cancer.

- Risk increases slightly if you’ve had a benign breast lump.

- Having had chest area radiation therapy in childhood or youth increases risk.

- Obesity is a possible risk, especially postmenopause.

- Diet high in saturated animal fats may increase risk.

- Compared to non-drinkers, women who consume one alcoholic drink daily may have a small increased risk; those who have two-five drinks daily have 1.5 times the risk.

- Exposure to oestrogen increases risk. Oestrogen stimulates cell division: the more cells divide, the more likely some may become cancerous.

- Women with early onset of menstruation, late menopause, a menstrual cycle shorter or longer than average, no pregnancies or first pregnancy after 30, have slightly higher risk.

- Oral contraceptives may slightly increase risk. Women who stopped using oral contraceptives over 10 years previously don’t appear to have increased risk.

- Over eight years of hormone replacement therapy (HRT) increases risk. Risk returns to that of the general population within five years of stopping HRT.

- Smoking may increase risk.

Who gets it?
As more women have mammograms, and with improved detection, rates of new cases of breast cancer have increased. The rate of death from all types of breast cancer hasn’t increased, however, as treatment is increasingly effective. It remains the most common cancer in women.

The disease is more common in older women, urban areas, higher socio-economic groups, unmarried women and Jewish women. Caucasians (especially of northern European descent) have slightly higher risk. Asian and Hispanic women have lower risk. Incidence in black women, specifically of African descent, is increasing.