The specific cause of breast cancer is unknown and there are known risk factors. Most of the risk factors (approximately 90%) are environmental and life-style related; while the rest (5% to 10%) are genetic.
Major risk factors
- Sex: Females have a 100 times higher risk of getting breast cancer than males (Female: male = 100:1)
- Age: although breast cancer can occur at any age, it is more common in older women. 70% of breast cancer patients diagnosed are within 40 – 69 years of age. Peak age range is 45 – 65 years
- Breast density: Dense breast means presence of more glandular tissue compared to the fat in the breast. (Breast density is a mammographic finding and cannot be reliably defined by a physical examination). Women with extremely dense breasts have a two-fold increased risk compared to women with breasts of average density. Usually, younger women are more likely to have dense breasts than older women. However, there are outliers at both ends of the age spectrum.
- Family history of breast or ovarian cancer – Strong family history indicates a risk of gene mutation in the family. Having one first degree relative with breast cancer is associated with excess incidence of breast cancer of 5.5% and if two first-degree relatives affected, the risk is increased up to 13.3%. The increase in risk is greater when the relative was affected at a younger age. Maternal and paternal sides of the family are equally important
- Known mutations – Women who are born with specific abnormal genes are at higher risk of developing breast cancer. (In a population-based series of breast cancer cases it has shown that approximately 1 -2% of women with breast cancer has BRCA 1 or BRCA 2 mutations Men and women can pass these abnormal genes to their children and transmission is autosomal dominant, so each child has a 50/50 chance of inheriting these gene mutations. It is important to assess history of cancer in maternal and paternal sides of the family.
Medical history and medications
- Previous breast cancer, Ductal carcinoma in situ, Lobular carcinoma in situ or proliferative breast disease can increase the chance of developing breast cancer during the lifetime. Risk of breast cancer will increase by at least four-fold in women with biopsy proven atypical hyperplasia or Lobular carcinoma in situ. This risk will persist for at least 25 years.
- Chest wall radiation: Women with a history of chest wall radiation for treatment of another disease condition have up to 10-fold-increased risk of breast cancer. The risk varies with the age of the woman when she had radiation therapy. Risk is highest if the woman had radiation before menarche.
- Hormone Replacement Therapy: Women who are using combined estrogen-progesterone Hormone Replacement Therapy (HRT), are at higher risk of developing breast cancer. Risk increases with the length of HRT use. After five years of using combined HRT, the risk of breast cancer increases by about 15% and the risk returns to baseline within about two years of stopping HRT. Estrogen therapy alone increases breast cancer risk but the increased risk is lower than for combined therapy.
- Oral Contraceptive Pills (OCP): Slight increased risk is observed in current users especially long-term users. Rapidly decreases the risk after stopping OCP. Factors related to reproductive system are;
- Hormonal influences: Menarche and Menopause: Women with earlier age of menarche (before the age of 11 years) and/or later age of menopause (after the age of 55 years) have an increased risk of breast cancer, mediated in part by the increased number of menstrual cycles and the longer lifetime exposure to estrogen and progesterone.
- Reproductive History: Nulliparity increases a woman’s risk of breast cancer, and every live birth reduces the relative risk by about 7%. Women 30 years or older at the time of their first childbirth have a higher risk of breast cancer than women having their first childbirth at a younger age.
- Breastfeeding: The relative risk of breast cancer decreases by about 4% for every month of breastfeeding. Reduced lifetime exposure to estrogen and progesterone may explain the protective effect conferred by breastfeeding.
Factors related to Lifestyle
- Overweight /Obesity – High BMI is associated with higher risk of post – menopausal breast cancer. It also negatively affects the prognosis of early-stage breast cancer. Weight gain – Increased risk in post - menopausal breast cancer. Increased weight → increased circulating estrogen
- Physical activity: Compared to the least active women breast cancer risk is reduced by 25% among women who are physically active.
- Alcohol consumption: Regular consumption of alcohol as little as one drink per day increases the risk of breast cancer by 4%.
- Smoking: Association between active smoking and breast cancer is consistent with causality. In addition, the association between secondhand smoke and breast cancer among younger, primarily premenopausal women who have never smoked is also consistent with causality.
Cancer Leaflet (PDF download)