NSABP B-51 / RTOG 1304

Design, population and eligibility

Treatment and comparator

Regional nodal irradiation versus no regional nodal irradiation. After lumpectomy, the comparison was whole-breast irradiation with or without regional nodes; after mastectomy, chest-wall and regional nodal irradiation versus no RT according to randomization. RT was delivered using protocol-specified conventional fractionation.

Primary and secondary endpoints

Primary endpoint: invasive breast cancer recurrence-free interval (IBCRFI), including invasive recurrence or breast-cancer death. Secondary endpoints included locoregional recurrence-free interval, distant recurrence-free interval, DFS, OS and safety.

Key efficacy results

After median follow-up 59.5 months, 109 primary endpoint events occurred (50 RNI; 59 no RNI). Five-year IBCRFI was 92.7% with RNI versus 91.8% without; HR 0.88 (95% CI 0.60–1.28; P=0.51). No significant benefit was observed in locoregional recurrence-free interval, distant recurrence-free interval, DFS or OS.

Source-reported outcome visualization

Five-year invasive breast cancer recurrence-free interval.

Negative superiority trial. The result does not establish formal equivalence or non-inferiority; event count and follow-up remain important.

Absolute effect

Five-year IBCRFI differed by 0.9 percentage points (92.7% vs 91.8%), with HR 0.88 (95% CI 0.60–1.28; P=0.51). No NNT is assigned because superiority was not demonstrated and the confidence interval permits clinically relevant benefit or harm. Do not describe this as proven equivalence.

Trial-specific toxicity and treatment burden

Grade 3 adverse events were approximately 10.0% with RNI versus 6.5% without; grade 4 events 0.5% versus 0.1%. No treatment-related deaths or unexpected safety signals were reported. Radiation dermatitis was the most common grade 3 toxicity. The toxicity profile must be weighed against the low absolute recurrence event rate.

Quality of life and patient-reported outcomes

No significant QoL advantage has been established. Avoiding regional RT reduces treatment exposure and potential late effects, but individual risks and baseline recurrence risk remain relevant.

Subgroups and applicability

The study included patients treated with lumpectomy and mastectomy, HR-positive and triple-negative/HER2-positive subtypes. Subgroup analyses did not establish a reliable group with a clear RNI benefit; interpret exploratory subgroup findings cautiously.

Current-practice context · reviewed 3 November 2026

For biopsy-proven cN1 patients converting to ypN0 after neoadjuvant systemic therapy, B-51 supports considering omission of RNI in selected patients after multidisciplinary review. It does not apply to initial cN2–3 disease, residual ypN-positive disease or all post-neoadjuvant settings.

Limitations and critical interpretation

The primary analysis had 109 events versus 172 planned; median follow-up was 59.5 months. A negative superiority test is not an equivalence test. Longer follow-up may refine late recurrence estimates, especially in HR-positive disease.

Bottom line: B-51 did not show a significant benefit from RNI in cN1 patients who converted to ypN0 after neoadjuvant chemotherapy. It supports response-guided de-escalation discussion but does not prove that RNI is unnecessary for every patient.

Primary sources and follow-up publications

1. Primary report, NEJM 2025
2. ClinicalTrials.gov
3. 2026 ASCO locoregional management review

Outcome values are sourced from the linked reports. Unreported values are not imputed. NNT/NNH is only stated when an explicit fixed-time absolute difference supports a defensible calculation; no NNT is derived from median survival.