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Breast Cancer Health Center

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    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 With Adequate Evidence of Increased Risk of Breast Cancer

    Sex and age

    Based on solid evidence, female sex and increasing age are the major risk factors for the development of breast cancer.

    Magnitude of Effect: Women have a lifetime risk of developing breast cancer that is approximately 100 times the risk for men. The short-term risk of breast cancer in a 70-year-old woman is about ten times that of a 30-year-old woman.

    Study Design: Many epidemiologic trials.
    Internal Validity: Good.
    Consistency: Good.
    External Validity: Good.

    Major inheritance susceptibility

    Based on solid evidence, women who inherit gene mutations associated with breast cancer have an increased risk.

    Magnitude of Effect: Variable, depending on gene mutation, family history, and other risk factors affecting gene expression.

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

    Breast density

    Based on solid evidence, women with dense breasts have an increased risk of breast cancer. This is most often an inherent characteristic, to some extent modifiable by reproductive behavior, medications, and alcohol.[1]

    Magnitude of Effect: Women with dense breasts have increased risk, proportionate to the degree of density. This increased relative risk (RR) ranges from 1.79 for women with slightly increased density to 4.64 for women with very dense breasts, compared with women who have the lowest breast density.

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

    Modifiable Factors With Adequate Evidence of Increased Risk

    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: Approximately a 26% increase in incidence of invasive breast cancer; the number needed to produce one excess breast cancer is 237.

    Study Design: Randomized controlled trials (RCTs). Furthermore, cohort and ecological studies show that cessation of combination HT is associated with a decrease in rates of breast cancer.
    Internal Validity: Good.
    Consistency: Good.
    External Validity: Good.

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