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Breast Cancer Prevention (PDQ®): Prevention - Health Professional Information [NCI] - Overview

Note: Separate PDQ summaries on Breast Cancer Screening; Breast Cancer Treatment; Male Breast Cancer Treatment; Breast Cancer Treatment and Pregnancy; and Levels of Evidence for Cancer Screening and Prevention Studies are also available.

Factors Associated With Increased Risk of Breast Cancer

Combination hormone therapy

Based on solid evidence, combination hormone therapy (HT; estrogen-progestin) is associated with an increased risk of developing breast cancer.

Magnitude of Effect for Combination Therapy: Approximately a 26% increase in incidence of invasive breast cancer; number needed to harm for every 237 patients participating in the Women's Health Initiative (WHI) trial and randomly assigned to the combination HT arm: 1 invasive breast cancer occurred beyond those that happened in the placebo arm of the trial.

Study Design: Randomized controlled trial (RCT) supported by evidence from cohort and ecological studies showing withdrawal of combination HT is associated with a decrease in the rates of breast cancer.
Internal Validity: Good.
Consistency: Good.
External Validity: Good.

Ionizing radiation

Based on solid evidence, exposure of the breast to ionizing radiation is associated with an increased risk of developing breast cancer, starting 10 years after exposure and persisting lifelong. Risk depends on dose and age at exposure, with the highest risk occurring during puberty.

Magnitude of Effect: Variable, but approximately a sixfold increase in incidence overall.

Study Design: Cohort or case-control studies.
Internal Validity: Good.
Consistency: Good.
External Validity: Good.

Obesity

Based on solid evidence, obesity is associated with an increased breast cancer risk in postmenopausal women who have not used HT. It is uncertain whether reducing weight would decrease the risk of breast cancer.

Magnitude of Effect: The WHI observational study of 85,917 postmenopausal women found body weight to be associated with breast cancer. Comparing women weighing more than 82.2 kg with those weighing less than 58.7 kg, the relative risk (RR) was 2.85 (95% confidence interval [CI], 1.81–4.49).

Study Design: Observational study.
Internal Validity: Good.
Consistency: Good.
External Validity: Good.

Alcohol

Based on solid evidence, exposure to alcohol is associated with an increased breast cancer risk in a dose-dependent fashion. It is uncertain whether decreasing alcohol exposure would decrease the risk of breast cancer.

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