Breast Cancer · Medical · DCIS · Adjuvant Endocrine Therapy · 2026-09-18

NSABP B-24

After lumpectomy and whole-breast RT for DCIS, does 5 years of tamoxifen reduce breast-cancer events?

Population

1,804 women with DCIS, including patients with involved specimen margins, treated with lumpectomy and 50 Gy RT.

Intervention

Tamoxifen 20 mg daily for 5 years.

Comparator

Placebo for 5 years.

Sample size / primary endpoint

1,804 randomized women; Breast-cancer events at 5 years

Absolute Effect

Primary efficacy result

5-year breast-cancer events: 8.2% vs 13.4% (−5.2 percentage points)

Median follow-up 74 months in the primary publication.

No meaningful overall-survival advantage was expected or demonstrated; breast-cancer mortality was uncommon.

Tamoxifen reduced ipsilateral and contralateral breast events; 5-year invasive breast-cancer events were 4.1% in the tamoxifen group.

NNT / NNH

NNT: 20, based on 5-year breast-cancer events.

NNH: not quantifiable for a single trial-specific principal harm; endometrial cancer and thromboembolic events were recognized tamoxifen-associated risks.

Treatment burden and toxicity

Tamoxifen adds endocrine toxicity, including thromboembolic events, endometrial cancer and cataracts. Endometrial cancer incidence was higher with tamoxifen in long-term analyses.

Quality of life: Quality of life was not the primary endpoint; toxicity and adherence are clinically relevant when applying the result.

Subgroups / clinically important modifiers

Later analyses indicate the invasive-event benefit is concentrated in ER-positive DCIS, supporting biomarker-informed use.

Limitations and maturity

The trial predates modern genomic/risk stratification and used tamoxifen 20 mg for 5 years. Absolute benefit depends on baseline recurrence risk.

Clinical interpretation

Tamoxifen adds a modest but measurable reduction in breast-cancer events after lumpectomy plus RT for DCIS, with benefit concentrated in ER-positive disease and balanced against endocrine toxicity.

B-24 established endocrine therapy as an additional risk-reduction layer after local therapy for DCIS rather than a replacement for local treatment.

Primary / authoritative sources

NSABP B-24 primary publication (PubMed)

Long-term B-17/B-24 recurrence analysis (PubMed)