On the Lookout:
Breast Cancer Risk Factors

By, Susan E. Appling, RN, MS, CRNP

A thorough understanding of risk factors is necessary for effective risk assessment and risk reduction counseling in the primary and secondary prevention of breast cancer. In general, women have a 12% (1 in 8) lifetime risk of developing breast cancer. An individual's risk may increase or decrease depending on the presence of factors that affect this overall risk.

Non-Modifiable Risk Factors

Many risk factors cannot be changed and can significantly impact breast cancer risk, making a thorough assessment of these factors paramount.

Age: Increasing age is a significant risk factor. Over 75% of all breast cancers occur in women over fifty. The probability of developing invasive breast cancer before the age of 40 is 1 in 235, between ages 40 and 59 it is 1 in 25 and between 60 and 79 it is 1 in 15. Generally, the longer a woman lives without breast cancer, the lower her risk of developing this cancer.

Gender: Women are nearly 100 times more likely to develop breast cancer than men. This year, approximately 1,400 new cases will be diagnosed in men compared to 182,800 in women. This gender difference is linked to hormonal influences.

Race: Breast cancer is highest in white women (113/100,000), followed closely by black women (99/100,000). Asian/Pacific Islander, Latino, and Native American women face a lower risk (77, 69 and 34/100,000, respectively).

Genetics: Five to ten percent of all breast cancers can be attributed to a mutation in the BRCA1 or BRCA2 cancer susceptibility gene. These genetic mutations are often found in families with early onset breast cancer (<50), bilateral breast cancer, breast and ovarian cancer, or multiple breast cancer cases. It is also more commonly found in families of Ashkenazi Jewish descent. Women with a BRCA1 or BRCA2 mutation have approximately an 80% lifetime risk of developing breast cancer and each offspring has a 50% chance of inheriting one of these autosomal dominant mutations.

Family History:Approximately 15-20% of all breast cancers are related to a family history of breast cancer, but lack evidence for strong genetic linkage through autosomal dominant transmission. In these family clusters, breast cancer may be due to a variety of factors including a weaker genetic predisposition, lifestyle, and/or environmental factors. Familial breast cancer risk can come from either maternal or paternal relatives. The degree of risk varies by the type and number of relatives affected. A positive family history in a second degree relative (aunt, grandmother) increases a woman's risk by 1.5. A positive family history in one first degree relative (mother, sister, daughter) doubles her breast cancer risk, while a positive family history in two first degree relatives increases her risk five fold.

Personal History: Previous history of breast cancer increases the risk of developing breast cancer in the contralateral breast by 3-4 fold.

Previous breast biopsies: A breast biopsy revealing atypical hyperplasia increases breast cancer risk by at least four fold, while those showing proliferative changes without atypia increase risk by only 50 to 100%. Simply having had one or more breast biopsies regardless of the histologic findings can increase breast cancer risk by about 50%.

Previous chest radiation: Women with a history of chest radiation for treatment of cancers such as Hodgkin's disease or non-Hodgkin's lymphoma face a two to four-fold increased risk.

Hormonal Influences: The impact of estrogen on breast cancer risk has been evaluated extensively and continues to be controversial. It is clear that both endogenous and exogenous estrogen as well as progesterone promote growth of breast tissue. This increased growth can result in increased DNA damage, mutations, and cancer. This process supports the hypothesis that the longer breast tissue is exposed to estrogen the greater the carcinogenic impact. There are several risk factors, both non-modifiable and modifiable, associated with this hormonal influence.

Early menarche: Menarche before the age of twelve increases breast cancer risk by about 20%.

Late menopause: Menopause occurring after age 55 doubles breast cancer risk.

Modifiable Risk Factors

Hormonal Influences: The following risk factors have a hormonal basis and, to some degree, are under individual influence.

Pregnancy: When the menstrual cycle is interrupted, particularly at an early age, breast cancer risk is reduced. Pregnancy before the age of twenty decreases breast cancer risk by about 20%, while pregnancy after 30 or nulliparity slightly increases risk. The number of full-term pregnancies are inversely proportional to breast cancer risk.

Lactation: Because breast feeding decreases the number of menstrual cycles, breast cancer risk may be decreased slightly, particularly for women who breast feed for at least 1.5 to 2 years.

Oophorectomy: For women with a BRCA1 mutation, early menopause that results from prophylactic removal of the ovaries decreases breast cancer risk by 47%.

Oral Contraceptives (OC): There is only a small increase in breast cancer risk with OC use, and it returns to normal 10 years post-use.

Hormone Replacement Therapy (HRT): HRT (including both estrogen only or estrogen and progestin replacement therapy) may annually increase breast cancer risk by about 2% for each year of use. Long-term use (over 10 years) of HRT may increase risk by about 30-40%. Risk decreases after stopping HRT and returns to that of the general population 5 years after stopping therapy.

Diet: Although many studies have not revealed convincing evidence that lower dietary fat intake results in decreased breast cancer risk, it has been found that women consuming a diet low in fat and high in fruits, vegetables, whole grains, and lean meats may have a 23 to 40% reduction in cancer-related deaths. The Women's Health Initiative, a randomized placebo-control primary prevention trial of over 47,000 women, is currently investigating the impact of diet on breast cancer risk, and results are expected in five years.

Body Weight: Obesity, particularly postmenopausal, seems to increase risk by increasing the amount of estrogen made outside the ovaries.

Exercise: Regular exercise may decrease breast cancer risk by 20 to 50%, although current studies often do not control for confounding variables.

Alcohol: Although results from studies vary, breast cancer risk seems to increase as alcohol intake increases. Consuming 2-5 drinks per day may result in a 40-50% increase in risk.

Environmental Factors:

Pollutants such as pesticides and polychlorinated biphenyls (PCBs) have been implicated in breast cancer although research remains inconclusive.

References

American Society of Clinical Oncologists. 1998. ASCO Curriculum.

CA: A Cancer Journal for Clinicians. Vol 50, No. 1, 2000. Cancer Statistics 2000.

Lobo, RA. (1999). Treatment of the Postmenopausal Woman. Lippincott Williams & Wilkins.