Reevaluating regional nodal irradiation after nodal pathologic complete response to neoadjuvant chemotherapy: toward individualized de-escalation

Introduction

The NSABP B-51/RTOG 1304 trial by Mamounas et al. (1) represents a landmark in the evolving paradigm of de-escalation in breast cancer treatment. This rigorously designed, multicenter, phase 3 randomized controlled trial asked a clinically relevant question: can regional nodal irradiation (RNI) be safely omitted in patients who present with node-positive breast cancer but convert to nodal pathologic complete response (pCR) following neoadjuvant chemotherapy (NAC)?

The results of this study—conducted in 1,641 patients with T1–3N1M0 breast cancer undergoing NAC with a median follow-up of 59.5 months—suggest that omission of RNI in this setting does not compromise event-free survival in the overall study population. These findings have significant implications for patients with human epidermal growth factor receptor 2 (HER2)-positive and triple-negative breast cancer (TNBC), subtypes in which NAC is routinely used and pCR is more common. However, a more granular look at the data raises several critical caveats regarding long-term outcomes and biological plausibility that warrant careful scrutiny before widespread practice change is adopted.

Methodological strengths

The design and execution of this trial are exemplary. Stratified randomization based on key biological and clinical variables, rigorous quality assurance in radiation delivery, high protocol adherence, and appropriate intention-to-treat analysis provide confidence in the integrity of the results. The inclusion of both mastectomy and breast-conserving surgery patients increases the generalizability of the findings.

Limitations: power, follow-up, and subgroup signals

Despite these strengths, several limitations temper the interpretation of the results. First, the trial experienced a substantially lower-than-expected event rate, with only 109 invasive breast cancer recurrence-free interval (IBCRFI) events observed versus the 172 planned. As a result, the final analysis was time-driven rather than event-driven, reducing statistical power and increasing the risk of a type II error.

Second, the median follow-up of just under 5 years may be insufficient for hormone receptor (HR)-positive known for a long tail of late recurrence (2). Notably, subgroup analysis showed higher recurrence in the no-RNI arm (9.8% vs. 4.5%) and a hazard ratio favoring RNI of 0.41 [95% confidence interval (CI): 0.17–0.99], suggesting a potential benefit that may become more pronounced with time.

Conversely, in the TNBC subgroup, recurrence rates were paradoxically higher in the RNI arm (hazard ratio =2.3; 95% CI: 1.0–5.25). This biologically intriguing result may reflect the detrimental effect of RNI on regional immune surveillance. Emerging data, such as from Chen et al. (3), demonstrate that immune-activated regional lymph nodes correlate with better outcomes in early TNBC. Disruption of nodal immune architecture through RNI may impair anti-tumor immunity, providing a plausible explanation for this unexpected finding.

Clinical implications: personalized de-escalation

The study provides high-quality evidence supporting the omission of RNI in selected patients with node-positive HER2-positive or TNBC who achieve nodal pCR after NAC. Among important secondary findings, patients who underwent sentinel lymph node biopsy (SLNB) alone derived no significant benefit from RNI, despite known false-negative rates associated with SLNB after NAC. We recently reported a 15.2% false-negative rate for SLNB alone, compared to targeted axillary dissection, now considered the gold standard for axillary staging post-NAC (4).

These findings reinforce a paradigm shift toward individualized, response-adapted therapy. However, the biologic heterogeneity of breast cancer warrants caution. Grouping molecular subtypes under a single randomization framework risks obscuring subtype-specific risks and benefits. Subgroup signals—especially when biologically plausible—should inform nuanced clinical decision-making.

The path forward

This trial advances the field by validating RNI omission in HER2-positive and TNBC patients achieving nodal pCR after NAC for T1–3N1M0 breast cancer. However, results for the HR⁺/HER2⁻ subgroup call for caution and longer follow-up to evaluate late recurrences.

Furthermore, the possible adverse impact of RNI on immune surveillance in TNBC underscores the need for translational studies examining immunologic consequences of nodal irradiation. Future trials should incorporate immunologic endpoints to understand interactions between radiation and immune response, especially as immune checkpoint inhibitors gain traction in TNBC.

Furthermore, randomised controlled trials are required to evaluate whether RNI can be safely omitted in patients presenting with cT4 and/or cN2/3 disease who achieve pCR following neoadjuvant systemic therapy.

For patients not achieving nodal pCR, results from the Alliance A011202 trial are anticipated to inform axillary management and whether axillary lymph node dissection can be safely replaced with radiation therapy to the undissected axilla and regional lymph nodes (5).

Conclusions

Mamounas et al. (1) have delivered high-quality evidence supporting the safe omission of RNI in selected patients (cT1–3N1M0, HER2⁺ or TNBC) with nodal pCR after NAC. However, the trial’s limited power, relatively short follow-up, and biologically plausible subgroup differences call for cautious interpretation in patients with HR⁺HER2⁻ disease. Personalized, biologically informed de-escalation—not universal RNI omission—represents the optimal path forward.

Acknowledgments

None.

Footnote

Provenance and Peer Review: This article was a standard submission to the journal. The article has undergone external peer review.

Peer Review File: Available at https://gs.amegroups.com/article/view/10.21037/gs-2025-240/prf

Funding: None.

Conflicts of Interest: Both authors have completed the ICMJE uniform disclosure form (available at https://gs.amegroups.com/article/view/10.21037/gs-2025-240/coif). K.M. received fees from Merit Medical and Q Medical Technologies corporations. The other author has no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

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References

1. Mamounas EP, Bandos H, White JR, et al. Omitting Regional Nodal Irradiation after Response to Neoadjuvant Chemotherapy. N Engl J Med 2025;392:2113-24. [Crossref] [PubMed]
2. Ito M, Amari M, Sato A, et al. Risk factors for late recurrence and postrelapse survival in estrogen receptor (ER)-positive, human epidermal growth factor receptor (HER) 2-negative breast cancer after 5 years of endocrine therapy. Breast 2024;73:103604. [Crossref] [PubMed]
3. Chen YY, Ge JY, Ma D, et al. Immune-Activated Regional Lymph Nodes Predict Favorable Survival in Early-Stage Triple-Negative Breast Cancer. Front Oncol 2020;10:570981. [Crossref] [PubMed]
4. Varghese J, Patani N, Wazir U, et al. Wire-Free Targeted Axillary Dissection: A Pooled Analysis of 1300+ Cases Post-Neoadjuvant Systemic Therapy in Node-Positive Early Breast Cancer. Cancers (Basel) 2024;16:2172. [Crossref] [PubMed]
5. Alliance for Clinical Trials in Oncology. A011202: A randomized phase III trial evaluating the role of axillary lymph node dissection in breast cancer patients (cT1-3 N1) who have positive sentinel lymph node disease after neoadjuvant chemotherapy. Available online: https://clinicaltrials.gov/ct2/show/NCT01901094