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


Magnitude of Effect: The RR of breast cancer is decreased 4.3% for every 12 months of breast-feeding, in addition to 7% for each birth.[1]

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

Interventions Associated With Decreased Risk of Breast Cancer

Selective estrogen receptor modulators (SERMs): Benefits

Based on solid evidence for tamoxifen and fair evidence for raloxifene, treatment reduces the incidence of breast cancer in postmenopausal women. Tamoxifen also reduced the risk of breast cancer in high-risk premenopausal women. The effects observed for tamoxifen show persistence several years after discontinuing active treatment.

Magnitude of Effect: Treatment with tamoxifen reduced breast cancer by about 50%. Treatment with raloxifene has a similar effect on reduction of invasive breast cancer but appears to be less effective for prevention of noninvasive tumors.

Study Design: RCTs.
Internal Validity: Good.
Consistency: Good.
External Validity: Good.

Selective estrogen receptor modulators (SERMs): Harms

Based on solid evidence, tamoxifen treatment increases the risk of endometrial cancer, thrombotic vascular events (pulmonary embolism, stroke, deep venous thrombosis), and cataracts. Many of these risks, notably pulmonary embolism and deep venous thrombosis, are reduced after discontinuing active treatment with tamoxifen. Based on fair evidence, raloxifene also increases venous pulmonary embolism and deep venous thrombosis but not endometrial cancer.

Magnitude of Effect: Meta-analysis showed RR was 2.4 (95% CI, 1.5–4.0) for endometrial cancer and 1.9 (95% CI, 1.4–2.6) for venous thromboembolic events.

Study Design: RCTs.
Internal Validity: Good.
Consistency: Good.
External Validity: Good.

Aromatase inhibitors or inactivators: Benefits

Based on solid evidence, aromatase inhibitors or inactivators (AIs) reduce the incidence of new breast cancers in postmenopausal women who have an increased risk of breast cancer.

Magnitude of Effect: After a median follow-up of 35 months, women aged 35 years and older who had at least one risk factor (e.g., women aged 60 years and older or those having a Gail 5-year risk >1.66% or ductal carcinoma in situ with mastectomy) and who took 25 mg of exemestane daily were less likely to be diagnosed with invasive breast cancer (hazard ratio [HR], 0.35; 95% CI, 0.18–0.70). The absolute risk reduction was 21 cancers avoided out of 2,280 participants over 35 months (number needed to treat [NNT], about 100).

Study Design: One RCT.
Internal Validity: Good.
Consistency: One study in women with no history of breast cancer, but consistent with RCTs in women with history of breast cancer.
External Validity: Good for women who meet inclusion criteria.
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