RNI Precision Trial Protocol: Optimization of Regional Node Irradiation for Sentinel Lymph Node-positive Breast Cancer Omitting Axillary Dissection Based on Clinical and Genomic Risk Assessment: A Prospective Clinical Trial

Abstract

Background

Sentinel lymph node biopsy (SLNB) has been the standard treatment procedure for clinically node-negative (cN0) breast cancer (BC). Three randomized trials (Z0011, AMAROS and SENOMAC) have shown that patients with low-burden sentinel lymph node metastasis can be safely omitted axillary lymph node dissection who receive adjuvant radiotherapy. However, these published studies have been insufficient to accurately assess the recurrence risk in this patient population, leading to variability in adjuvant radiotherapy volume across different studies.

Objective

This study evaluates whether an integrated axillary management strategy combining SLNB with individualized regional nodal irradiation (RNI) based on recurrence risk reduces 2-year lymphedema compared to ALND followed by comprehensive RNI.

Materials and Methods

This trial is a single institute, open-labeled, non-randomized cohort trial. Participants are stratified into two cohorts based on the extent of axillary surgery after enrollment. For SLNB cohort, patients are divided into three groups according to clinical and genomic risk assessment. Clinically high-risk patients are defined as having at least two factors (tumor size≥2cm, percent of positive SLNs>30%, LVI positive and SLN macro-metastases), who have a predicted risk of n-SLN involvement greater than 30%. Clinically high-risk n-SLN involvement patients are further detected by using RecurIndex test. For pathological node positive (pN+) after ALND cohort, patients are treated with WBI/chest wall irradiation combined with comprehensive RNI excluding the dissected axillary region; A total of 205 patients will be enrolled, with 68 patients in SLNB cohort and 137 patients in ALND cohort with a 1:2 ratio assignment. The RNI precision trial uses a clinical-genomic model to accurately stratify the recurrence risk and provides the individualized RNI volume for early-stage BC with low-burden sentinel lymph node metastasis, and we anticipate that our study will provide high-quality data to potentially support individualized RNI in this patient population.

Keywords

sentinel lymph node clinical-genomic model breast cancer lymphoedema regional node irradiation omitting axillary dissection

Introduction

According to the report of the National Central Cancer Registry (NCCR) of China, breast cancer (BC) was the most common cancer and the fifth cause of cancer deaths among women in China in 2015, with 303,600 newly diagnosed cases and 70,400 breast cancer deaths.¹ Axillary lymph node dissection (ALND) is a crucial component of surgical treatment for BC, providing accurate staging of the axilla and regional control, which is vital for prognosis assessment and the selection of postoperative adjuvant therapies.^2,3 However, ALND also presents several issues. On one hand, the axillary lymph node metastasis rate in clinically node-negative patients is only 20% to 35%, meaning that 65% to 80% of patients undergo unnecessary surgery.^4,5 On the other hand, ALND can lead to various postoperative complications, such as upper limb lymphedema, pain, shoulder joint mobility impairment, numbness, and lymphatic leakage, significantly impacting the quality of life for breast cancer patients.^6,7 The NSABP-B32 study confirmed that for patients with negative sentinel lymph nodes (SLNs), the probability of non-sentinel axillary lymph node positivity is less than 10%, and further performing ALND did not yield benefits in terms of local recurrence or survival. Based on these findings, various guidelines both domestically and internationally, including those from the National Comprehensive Cancer Network (NCCN), the St. Gallen Consensus Conference, and the Chinese Anti-Cancer Association,⁸ now recommend SLNB as the standardized axillary surgical approach for clinically node-negative early-stage BC.

Subsequently, more research has focused on omitting ALND for BC patient with low-burden sentinel lymph node metastasis. The IBCSG23-01 study confirmed the feasibility of omitting ALND for BC patients with 1-2 micro-metastases (≤2mm) in sentinel lymph nodes.⁹ The ACOSOG Z0011 study further validated that for clinical T1-2 BC patients undergoing breast-conserving surgery, ALND was not necessary for those with 1-2 SLN macro-metastases. Compared to the SLNB alone group, there were no significant differences in 10-year disease free survival (80.2% vs. 78.2%) or overall survival (86.3% vs. 83.6%) between the SLNB and SLNB + ALND groups.¹⁰ The AMAROS study indicated that for early-stage BC patients with positive SLNB, axillary radiotherapy could be considered as an alternative to ALND. The 5-year axillary recurrence was 0·43% (95% CI 0·00–0·92) after axillary lymph node dissection versus 1·19% (0·31–2·08) after axillary irradiation.¹¹ In addition, the risk of lymphedema was 11% in the axillary radiotherapy, which was significantly lower than those in ALND group (23%, p<0.0001). More recently, the SENOMAC trial, enrolling the largest sample size of 2,766 BC patients, also confirmed that the omission of completion axillary-lymph-node dissection was noninferior to the more extensive surgery in patients with clinically node-negative breast cancer who had SLN macro-metastases after receiving regional nodal irradiation (RNI).¹²

Although all of these published studies reveal that RNI to the axilla can replace ALND in early-stage patients with SLN macro-metastases, the optimization of RNI fields for this patient population remains undetermined. In the Z0011 study, postoperative high-tangential whole-breast irradiation had been used in more than half the SLNB group and 19% of these patients treated with protocol-prohibited RNI. In the AMAROS study, RNI included the contents of all 3 levels of the axilla as well as the medial part of the supraclavicular fossa. In the SENOMAC trial, RNI to the supraclavicular nodes and/or level III axilla had been used for SLNB group, and treatment to the internal mammary chain was up to the treating institution. Therefore, in the era of precision irradiation, there is an urgent need to conduct high-quality clinical studies to accurately stratify the recurrence risk of breast cancer patients with low-burden positive sentinel lymph nodes, thereby optimizing the RNI strategies for this patient population. As a result, we design this prospective trial to evaluate an integrated axillary management strategy—SLNB with risk-adapted RNI—compared to the standard strategy of ALND followed by comprehensive RNI. The primary comparison is between these two integrated strategies, with the goal of reducing breast cancer-related lymphedema while maintaining oncologic safety. The research group write this protocol according to the Standard Protocol Items: Recommendations for Interventional Trials (SPIRIT) guidelines.¹³ The SPIRIT Checklist, 2013, is completed in Supplemental Table 1.

Methods and Analysis

Objectives

The primary endpoint of this prospective, non-randomized study with two cohorts aims to investigate whether an integrated axillary management strategy—SLNB with risk-adapted RNI based on recurrence risk using a combined clinical-genomic model—significantly decreases 2-year BCRL compared to the standard strategy of ALND followed by comprehensive RNI. It is important to note that the primary comparison in this study is between two integrated axillary management strategies, rather than isolating the effect of radiotherapy technique alone. The experimental strategy combines de-escalated surgery (SLNB) with risk-adapted RNI, and is compared against the current standard strategy of ALND followed by comprehensive RNI. This comparison reflects a pragmatic evaluation of contemporary treatment pathways, acknowledging that the choice of surgical approach (SLNB vs. ALND) is guided by clinical nodal status and institutional practice, not by randomization.

Trial Design

This is a prospective, open-labeled, non-randomized cohort trial. We recruit two cohorts histo-pathologically confirmed invasive breast cancer patients from Ruijin Hospital, Shanghai Jiaotong university school of medicine. We firstly developed a prediction model to assess the frequency of non-sentinel lymph node metastases after a positive sentinel lymph node based on four clinical-pathological factors¹⁴(tumor size≥2cm,^15,16 percent of positive SLNs>30%,^15,17,18 LVI positive^15,17 and SLN macro-metastases¹⁷). Patients presenting with two or more of the above four clinicopathologic factors are classified as clinically high-risk. The cut-off (≥2 factors) corresponds to a predicted risk of n-SLN involvement >30%, justifying escalation of adjuvant radiotherapy strategy.

The formalin-fixed and paraffin-embedded (FFPE) tumor samples of those clinically high-risk n-SLN involvement patients would be detected using RecurIndex test, which is a multi-genomic test based on Chinese 28 genes.^19,20 The RecurIndex (28-gene) assay, which will be used for genomic risk stratification in this study, has been validated for its prognostic value across major breast cancer biologic subtypes (including hormone receptor-positive, HER2-positive, and triple-negative disease) in prior studies.^20-22 We hypothesize that the risk of recurrence for early-stage SLN-positive BC patients might be accurately stratified by using a combined clinical-genomic model, and that individualized RNI in SLNB cohort could potentially reduce the risk of breast cancer-related lymphedema (BCRL) compared with SLNB+ALND cohort (Figure 1). While the primary endpoint is patient-reported and objective BCRL, this study is conducted under the overarching hypothesis that the de-escalated axillary management strategy (SLNB + personalized RNI) will yield non-inferior oncologic outcomes compared to the standard strategy (ALND + comprehensive RNI). The sample size provides preliminary but robust data on recurrence events to monitor this safety premise.

Figure 1.

The design of the RNI precision trial

Rationale for the Two-Tiered Risk Assessment

The selection of RNI volume in the SLNB cohort is guided by a two-tiered risk assessment strategy, each addressing a distinct clinical question:

(1) Clinical Model for Axillary Disease Burden: A clinical-pathological model (incorporating tumor size, proportion of positive SLNs, lymphovascular invasion, and SLN macro-metastasis) predicts the risk of additional disease in non-sentinel lymph nodes. This model stratifies patients into clinically low-risk (≤30% predicted risk of non-SLN metastasis) and clinically high-risk (>30% predicted risk) categories, primarily informing the decision on whether to include the undissected axilla (Level I-II) within the RNI target volume.

(2) Genomic Model for Overall Locoregional Recurrence (LRR) Risk: For patients classified as clinically high-risk, the RecurIndex (28-gene) assay is employed. This genomic tool predicts the intrinsic risk of LRR, independent of anatomical disease burden. Its value lies in further identifying a subset with genomically low LRR risk (7-year LRR <3%), who may be spared the most extensive RNI fields (e.g., omission of internal mammary node irradiation, IMNI).^22,23

Participants and Recruitment

The study was conducted in accordance with the Helsinki Declaration of 1975, as revised in 2024. Patients will be recruited by radiation oncologists in our institution. For each potential participant, the detailed instruction of present study is offered before enrollment, and each enrolled patient should sign the informed consent. A total of 205 patients will be enrolled, 68 patients in SLNB cohort and 137 patients in ALND cohort with a 1:2 ratio assignment. Baseline clinicopathological data will be prospectively collected for all enrolled patients. This will include, but is not limited to, age, tumor stage, surgical details, estrogen receptor (ER), progesterone receptor (PR), and HER2 status, and results of the RecurIndex genomic assay where applicable. All enrolled patients are clinically node-negative (cN0) at presentation. Cohort allocation is determined by the extent of axillary surgery performed following sentinel lymph node biopsy, not by upfront clinical suspicion of high axillary burden. This design feature ensures that both cohorts originate from the same baseline risk population, substantially reducing the risk of structural bias related to baseline disease burden.

Inclusion Criteria

(1) patients are able to understand and willing to participate the trial and signed consent form is obtained.

(2) histologically confirmed invasive breast carcinoma;

(3) clinical node-negative(cN0) BC treated with SLNB alone with SLNB involvement (SLNB cohort); or cN0 BC with pathological node positive (pN+) after ALND (active controlled cohort);

(4) for SLNB cohort, the risk of recurrence is stratified by using n-SLN metastasis clinical risk model (tumor size≥2cm, percent of positive SLNs>30%, LVI positive and SLN macro-metastases) combined genomic model;

(5) no BRCL is detected for each patient at baseline;

(6) female patients are aged between 18 and 80 years old;

(7) Pathologically surgical margin is negative (no tumor on ink);

Exclusion Criteria

(1) present with distant metastatic disease;

(2) involvement of ipsilateral supraclavicular lymph node;

(3) are pregnant or lactating women;

(4) are treated with neoadjuvant systematic therapies;

(5) History of non-breast malignancy within 5 years with the exception of lobular carcinoma in situ, basal cell carcinoma of the skin, carcinoma in situ of skin and carcinoma in situ of the cervix;

(6) have a history of radiotherapy to the neck, chest and/or ipsilateral axillary region;

Treatment

(1) SLNB Cohort

Step 1: Clinical Risk Stratification: All enrolled patients in the SLNB cohort are assessed using the 4-factor clinical model. Patients with 0 or 1 risk factor are classified as Clinically Low-Risk.

Step 2: Genomic Testing for Clinically High-Risk Patients: Patients with 2 or more clinical risk factors are classified as Clinically High-Risk and undergo RecurIndex testing on their primary tumor sample.

Step 3: Integrated Treatment Decision:

Group A (Clinically Low-Risk): Receive whole breast/chest wall irradiation (WBI/CWI) alone. Axillary RNI is omitted.

Group B (Clinically High-Risk & Genomically Low-Risk): Receive WBI/CWI + RNI encompassing the undissected axilla (Level I-II) and supraclavicular fossa, but omitting IMNI.

Group C (Clinically High-Risk & Genomically High-Risk): Receive WBI/CWI + Comprehensive RNI, including the undissected axilla, supraclavicular fossa, and IMNI.

Algorithm Note: In cases of discordance (e.g., clinical high-risk but genomic low-risk), the genomic risk assessment will supersede for decisions regarding the most extensive nodal coverage (i.e., IMNI inclusion), aligning with the principle of de-escalation when supported by low intrinsic biological risk. This approach is consistent with our ongoing IMNI PRECISION trial.²³

(2) ALND cohort

For pathological node positive (pN+) after axillary-lymph-node dissection (ALND) cohort, patients are treated with WBI/chest wall irradiation combined with comprehensive RNI excluding the axillary region (active controlled cohort).

Standardization of Surgical and Radiotherapeutic Practice

To ensure consistency in treatment delivery across this single-institution study, all interventions adhere to institutional standard operating procedures.

Surgical Management

The decision to perform SLNB or ALND follow institutional guidelines aligned with CSCO consensus. SLNB is indicated for clinically node-negative (cN0) patients. ALND is performed for patients with confirmed nodal metastasis meeting institutional criteria (≥3 positive sentinel nodes, gross extracapsular extension).

Radiotherapy Management

Localization, Simulation, and Immobilization

Patients are positioned supine using breast boards. Radio-opaque markers are routinely placed on the surgical scar, inframammary fold, and superior border of the breast tissue. A treatment planning CT scan will be performed with an image thickness of ≤ 3-5 mm. Skin bolus of 3 mm on the whole chest wall is recommended to use in case of mastectomy and be documented as well as evaluated within the quality assurance programmer of the study.

Target Volumes Definition

Gross target volume (GTV) of tumor bed is contoured according to metal markers. The CTVs include ipsilateral chest wall/breast with a tumor bed boost to the tumor bed for patients who underwent breast-conserving surgery. Clinical target volume (CTV) of tumor bed is expanded from GTV of tumor bed by 1 cm and is limited to the whole breast CTV. The planning target volumes (PTV) of whole breast and tumor bed are expanded from CTV by 0.5 cm in all directions and are limited to 0.5 cm beneath the skin surface. When indicated, regional lymph nodes of supraclavicular lymph nodes, undissected axilla, and internal mammary nodes are included. The regional nodes delineation is countered based on Ruijin guideline. The detailed delineation is listed in previous works.²⁴

The recommended dose constraints for OARs and required dose coverage of PTV are summarized in Supplemental Table 2 and Supplemental Table 3, which are inconsistent with our two previous trial protocols.^23,25

Radiotherapy Regimen

All enrolled patients are treated with hypo-fractionated radiotherapy. The hypo-fractionated prescribed dose is 40.05 Gy/15 Fx, with a tumor bed boost for breast conserving surgery (BCS) patients.

Outcomes

The Primary Outcomes

The primary outcome is 2-year BRCL, which is defined as incidence of any ipsilateral arm BRCL symptoms/signs after the completion of adjuvant radiotherapy. While optoelectronic perometry is a more sensitive objective tool, it is not routinely available in our clinical workflow. We employ a standardized circumferential measurement protocol with fixed anatomical landmarks to ensure reproducibility in the present study.

(1) The Norman questionnaire is used to detect any patients’ self-reported BCRL signs and symptoms. The questionnaire has been found to a reliable and valid questionnaire for the characterization of BCRL using self-reported signs and symptoms.²⁶ The detailed questionnaire has been reported in our previous study.²⁷ The total score ranges from 0 to 9 points, and the diagnosis is 1–3 points for mild BCRL, 4–6 points for moderate BCRL, and 7–9 points for severe BCRL.

(2) Standardization of Arm Circumference Measurement: Arm circumference measurement is used for objective evaluation of upper limb lymphedema.^28,29 With the patient seated and the arm relaxed and extended on a table, the circumference will be measured at two predefined points: 10 cm proximal and 10 cm distal to the olecranon (tip of the elbow). Each measurement will be taken twice at each point; if the two readings differ by >0.5 cm, a third measurement will be taken. The average of the two closest readings at each point will be recorded for analysis. An increase of ≥2.0 cm at either point compared to the pre-radiotherapy baseline constitutes an objective BCRL event.³⁰ According to the LENT SOMA criterion, the severity of BCRL is graded as mild (2cm< circumference change≦ 4 cm), moderate (4cm< circumference change≦ 6 cm), and severe (>6 cm).³¹

The Secondary Endpoints Include the Following Outcomes

(1) disease-free survival, which is defined as the time from enrollment to the time of a first recurrence in ipsilateral chest wall or in breast or in regional nodal or distant sites, a contralateral BC, or breast cancer specific death;

(2) local regional recurrence free survival, which is defined as the time from enrollment to a first recurrence in ipsilateral chest wall or in breast or in regional nodal areas;

(3) overall survival, which is defined as the time from enrollment to death from any cause.

Patient-Reported Arm Disabilities

Patients’ subjective assessment of disability using the Quick Disability of the Arm, Shoulder, and Hand (Quick DASH) questionnaires. Quick DASH consists of 11 items, and is a self-rating scale of daily functioning including social functioning and physical activity and upper extremity symptoms. Each item is divided into 5 levels, and the patient’s score is calculated as score = [(patient score/number of response items) - 1] × 25, with a total score of 0 to 100, and the higher the score, the greater the degree of upper limb dysfunction. Patients will be followed up with questionnaires using Quick DASH at baseline, 6, 12, 18, 24 months postoperatively, and the Quick DASH scale will be used to assess changes in arm morbidity from baseline to 2 years after radiotherapy.

Sample Size

The sample size is calculated for the primary endpoint of 2-year BCRL incidence using a two-proportion Z-test (PASS 2021). The assumed 2-year BCRL rate in the control cohort (ALND + RNI) is 32.5%, based on our institutional retrospective data.²⁷ For the experimental cohort (SLNB + personalized RNI), we hypothesize a reduction to 15%. This effect size is considered clinically meaningful and is supported by: 1) the observed 10.8% rate in our SLNB+RNI patients, 2) temporal incidence patterns from prospective cohorts where the majority of BCRL cases are identified within 2 years post-surgery,²⁸ and 3) dosimetric evidence that personalized RNI can further reduce dose to lymphedema-critical substructures in our recent study.³² With a power of 80%, a two-sided alpha of 0.05, and an anticipated 5% dropout rate, the required total sample size is 205 patients (SLNB: ALND allocation ratio = 1:2). The sample size is calculated based on the anticipated difference between the two integrated management strategies (SLNB + personalized RNI vs. ALND + comprehensive RNI), rather than to isolate the effect of radiotherapy field modification alone. The study is designed to provide preliminary safety data on oncologic outcomes, with a focus on feasibility and hypothesis generation. The sample size provides adequate power only for the primary endpoint (BCRL); secondary oncologic endpoints will be analyzed descriptively with appropriate caution.

Data and Safety Monitoring

(1) Data and Safety Monitoring Board (DSMB): An independent DSMB will be established. The DSMB will consist of experts in breast oncology, radiation oncology, biostatistics, and bioethics who are not otherwise involved in the trial. The DSMB charter will detail its operating procedures.

(2) Stopping Rules for Safety: The DSMB will conduct interim analyses to review efficacy and safety data, with a focus on oncologic outcomes in the experimental (SLNB) cohort. Pre-defined stopping rules for safety will include but are not limited to: an observed axillary recurrence rate in the SLNB cohort exceeding 1.5% at any pre-specified interim analysis, with a rate exceeding 1.0% triggering an expedited safety review. These thresholds are based on historical control data from pivotal trials (e.g., AMAROS, SENOMAC) where axillary recurrence rates were approximately 1%.

(3) Review Schedule: The DSMB will meet at least annually, or more frequently as dictated by accrual rates or emerging safety signals.

Data Collection and Management

The data collection is conducted by clinical researchers under the supervision of the primary investigators (PIs), who are responsible for the accuracy, completeness, and timeliness of the clinical data. Clinical data are stored in an online clinical database built by Ruijin Hospital. A logical validation process is established when creating the database. The PIs, ethical committees, sponsors are allowed to access database for analyzing and data monitoring at any time.

All data and original documents in the study should be retained for a minimum of five years, and permission from the ethics committee must be obtained before any destruction.

Follow Up

After completion of radiotherapy, each enrolled patient is followed up for at least 5 years. According to the follow-up program, BRCL and the oncological outcomes will be assessed weekly during treatment and at each follow-up visit (every 6 months after RT within 5 years, and then annually) (Table 1). Any recurrence is cytologically or histologically confirmed whenever possible. According to the National Cancer Institute’s Common Terminology Criteria for Adverse Events version 5.0 (NCI-CTCAE 5.0) and the Radiation Therapy Oncology Group (RTOG) scale, acute and late radiation related toxicity during each follow-up visits are recorded and graded.

Table 1.

Shows the Follow-Up Schedule

Timepoint	Study period
	Pre-radiotherapy	During radiotherapy	Post-radiotherapy
	0	Weekly	6m	12m	18m	2y	3y	4y	5y
ENROLLMENT:
Eligibility screening	X
Informed consent	X
Randomization	X
INTERVENTIONS:
SLNB cohort		X
ALND cohort		X
ASSESSMENTS:
Physical Examination	X	X	X	X	X	X	X	X	X
upper limb lymphedema	X		X	X	X	X	X	X	X
upper limb dysfunction	X		X	X	X	X	X	X	X
Complete blood cell count	X	X	X	X	X	X	X	X	X
Thyroid function	X		X	X	X	X	X	X	X
Myocardial enzyme	X		X	X	X	X	X	X	X
Chest CT scan	X			X		X	X	X	X
Mammography				X		X	X	X	X
Ultrasound for Breast and regional nodes	X		X	X	X	X	X	X	X
ECG	X		X	X	X	X	X	X	X
Echocardiography	X		X	X	X	X	X	X	X

Statistical Analyses

Statistical analysis will be performed based on the intent-to-treat principle, with all patients analyzed as enrolled, regardless of eligibility or protocol compliance. Baseline characteristics, including demographic, clinicopathological (tumor size, grade, lymphovascular invasion, and ER/PR/HER2 status), and treatment-related variables, will be summarized descriptively for both the SLNB and ALND cohorts. Continuous variables will be presented as means with standard deviations or medians with interquartile ranges, and categorical variables as frequencies and percentages. The comparability of the cohorts will be assessed using appropriate tests. The clinical risk model is intended as a screening tool to identify patients who may benefit from further genomic risk assessment, rather than as a definitive predictor of non-SLN metastasis. The prespecified threshold of ≥2 risk factors (predicted non-SLN risk >30%) was selected to optimize sensitivity for detecting clinically meaningful disease burden, accepting moderate specificity, with the understanding that subsequent genomic testing will refine risk classification.

For the primary endpoint, cumulative incidence of 2-year BRCL would be calculated and compared between SLNB and SLNB+ALND cohorts. Survival outcomes will be analyzed both for the entire SLNB cohort (vs. ALND cohort) and within the SLNB cohort according to the final assigned treatment group (A, B, C as defined above). The association of the clinical risk score with axillary recurrence will be evaluated. The RecurIndex genomic risk category will be analyzed for its association with LRR, particularly in sites beyond the axilla. For locoregional recurrence, cumulative incidence functions will be estimated using a competing-risk framework, with death as a competing event. Gray’s test will be used for between-group comparisons. A pre-specified exploratory subgroup analysis will be performed to assess the consistency of treatment effects (for both the primary endpoint of 2-year BCRL and secondary oncologic outcomes) across key biologic subtypes defined by hormone receptor and HER2 status. Interaction tests will be used to evaluate potential effect modification by subtype. Comparisons within the SLNB cohort are explicitly designated as exploratory. These analyses aim to generate hypotheses regarding the independent effect of RNI field personalization when surgical extent is held constant. Given the limited sample size within these subgroups, results will be interpreted descriptively and with caution, focusing on effect sizes and confidence intervals rather than statistical significance alone. For each statistical analysis, a p value <0.05 is regarded as statistically significant. To evaluate the incremental contribution of RNI field design, a pre-specified exploratory analysis will be conducted within the SLNB cohort, comparing 2-year BCRL rates across the three treatment groups (A, B, C). This analysis holds surgical extent constant (SLNB alone) and may provide hypothesis-generating evidence regarding the relationship between RNI volume and lymphedema risk.

Control for Confounding in Cohort Comparisons

To address potential confounding due to the non-randomized allocation of surgical strategy, we will employ rigorous statistical methods, including propensity score-based approaches such as Inverse Probability of Treatment Weighting (IPTW). The propensity score will be estimated using a logistic regression model that includes key baseline covariates: age, tumor size, histologic grade, lymphovascular invasion (LVI), number of positive sentinel lymph nodes, biologic subtype (ER/PR/HER2 status), and use of systemic therapy (chemotherapy, endocrine therapy and/or targeted therapy). We will then use Inverse Probability of Treatment Weighting (IPTW) to create a balanced pseudo-population for the primary time-to-event analysis. Missing data for covariates used in the propensity score model will be addressed using multiple imputation by chained equations (MICE) for variables with missingness <10%. For variables with >10% missingness, a missing indicator approach will be used in sensitivity analyses. The balance of covariates after weighting will be assessed using standardized mean differences (absolute value <0.1 considered balanced). Sensitivity analyses using E-values will be performed to assess the minimum strength of association that an unmeasured confounder would need to have with both the treatment and outcome to explain away the observed effect.

Results

The trial has been approved by the Ethics Committee of Ethics Committee of Ruijin Hospital, Shanghai Jiaotong University school of medicine on 14 June 2024 and registered on ClinicalTrials.gov (NCT06583655) before recruitment. The currently used trial protocol version is Version 1.0 (November 2023). The recruitment of the trial was initiated on 20 August 2024. Written informed consent was obtained from all participants prior to their enrollment in the study. Currently, the RNI PRECISION trial remains at the recruiting stage and a total of 75 patients have been enrolled until 30 June 2025. The first patient was enrolled on the 2 September 2024 and the expected enrollment duration is 2 years. As the primary endpoint is a 2- year BRCL rate, the final data of collection for the primary outcome measure is expected to be September 2028.

Results from the primary and secondary endpoints are planned to be published in scientific peer- reviewed journals, and at scientific conferences. Important protocol modifications will immediately be communicated to relevant parties.

Discussion

Since the publication of Z0011 and AMAROS study, omitting ALND is an acceptable treatment option for early-stage BC patients with low-burden sentinel lymph node metastasis. More recently, SENOMAC trial with the largest sample size of 2766 patients, also confirms that the omission of ALND is noninferior to the ALND group in patients with cN0 BC who have SLN macro-metastases after receiving adjuvant RNI. The publication of these high-quality randomized clinical studies on BC patients with low-burden SLN metastasis has further promoted the implementation of omitting ALND in this population, which represents the ongoing efforts to reduce the complications associated with ALND in modern radiation treatment. This provides new options for the management of the axilla in patients with low-burden SLN metastasis BC, but it also introduces new challenges for radiation oncologists to choose the optimal adjuvant radiotherapy regimen.

The Z0011 study protocol required enrolled patients to receive breast tangent field radiotherapy. Among the 891 patients actually enrolled, only 228 (29%) could be traced for specific radiotherapy information. Of these, 185 patients (81.1%) received the tangent field radiotherapy as specified in the clinical trial protocol. Out of these, 142 patients (76.8%) were assessable for tangent field height, and 73 patients (51.4%) received high tangent field radiotherapy. At the same time, 18.9% of patients received additional regional node irradiation, and 7.9% even received axillary radiotherapy.³³ In the AMAROS study, the fields of axillary radiotherapy group included the drainage areas of axillary lymph nodes in levels I-III and the supraclavicular region. However, the results showed that the 5-year local recurrence rate in the axillary radiotherapy group was only 1.19%. As a result, there is a debate among clinical experts regarding the necessity of comprehensive regional node radiotherapy strategies for BC patients with low-burden SLB metastasis. In the SENOMAC trial, two RNI strategies, including axillary lymph nodes in levels I-III+ supraclavicular region and axillary lymph nodes in levels III+ supraclavicular region, are both acceptable. On the other hand, the results from the EORTC-22922 and MA-20 studies involving BC patients with 1-3 positive nodes (pN1) after ALND, who received adjuvant radiotherapy that includes the comprehensive regional node irradiation including the internal mammary region, can reduce the risk of disease-free survival and breast cancer-specific mortality.^34,35 Therefore, there is significant controversy regarding the adjuvant RNI fields for early-stage BC patients with low-burden SLN metastasis.

In this study, our research team retrospectively analyzed 392 early-stage BC patients who had 1-3 positive sentinel lymph nodes and underwent ALND at Ruijin Hospital between 2009 and 2019. Using a logistic regression model, we screened 17 factors potentially related to non-sentinel lymph node metastasis. We ultimately identified four independent predictive factors for non-sentinel lymph node metastasis based on positive sentinel lymph node biopsy: tumor size, proportion of positive sentinel lymph nodes, lympho-vascular invasion (LVI), and macro-metastasis of SLN (Figure 2A). Based on these four factors, we constructed a nomogram to predict non-SLN metastasis for SLN positive BC patients, and the model’s predictive AUC value was 0.695(Figure 2B). We used these four independent risk factors to predict the probability of non-sentinel lymph node metastasis in patients with positive sentinel lymph nodes (Table 2). A total of 392 patients were included in the analysis, with 83 (21.2%) presenting non-SLN metastasis. The results indicated that none of the 12 patients with zero risk factors had non-sentinel lymph node metastasis. For those with one risk factor, the probability of non-SLN metastasis was 22.6-32.8%; with two clinical high-risk factors, it was 27.5-36.4%; with three clinical high-risk factors, it was 28.7-37.9%; and for patients with four independent high-risk factors, the probability was 40.7%. These findings were externally validated in the Z0011 cohort, where the overall non-SLN metastasis rate was 27.5%, with consistent risk-stratified probabilities: 0% for zero risk factors (0/10), 30.3%–33.6% for one, 31%–40% for two, 33.3%–48% for three, and 46.8% for four risk factors. Based on these results, we defined patients with 0–1 risk factors as low-risk (predicted non-SLN metastasis ≤30%) and those with ≥2 risk factors as clinically high-risk (>30%).

Figure 2.

Non-sentinel lymph node metastasis prediction model

Table 2.

Internal and External Validation of Non-SLN Predictive Model According to Risk Factors

Group	Ruijin hospital data		Z-0011 trial
Group	Patients	Non-SLNpositive rate (%)	Patients	Non-SLNpositive rate (%)
Overall Non-SLN positive rate	83/392	21.2%	92/334	27.5%
4 risk factors	11/27	40.7%	15/32	46.8%
3 risk factors	-	28.7%-37.9%	-	33.3%-48%
2 risk factors	-	27.5%-36.4%	-	31%-40.8%
1 risk factors	-	22.6%-32.8%	-	30.3-37%
0 risk factors	0/12	0%	0/10	0%

We acknowledge that the clinical nomogram demonstrates only moderate discriminative accuracy (AUC 0.695), a limitation inherent to models based solely on clinicopathologic factors. Rather than undermining the study, this observation directly supports the rationale for incorporating genomic testing. In our two-tiered risk stratification strategy, the clinical model serves as a pragmatic first-stage screening tool to identify patients with potentially higher axillary disease burden. For those classified as clinically high-risk, we then apply the RecurIndex 28-gene genomic assay to refine risk classification based on intrinsic biological risk. Our ongoing trials have demonstrated that this approach can substantially reclassify risk: among clinically high-risk patients, approximately 44% were identified as genomically low-risk,²³ suggesting that a meaningful proportion may be spared from comprehensive RNI without compromising oncologic safety. This integrated clinical-genomic strategy directly addresses the limitations of any single modality and represents a pragmatic pathway toward precision irradiation. The claim of “precision irradiation” in this study rests on the integration of clinical and genomic risk assessment, not on the performance of the clinical model alone.

Compared to traditional clinical-pathological models, genomic models provide more accurate prognostic information for individualizing precision irradiation for cancer patients. Currently, three commonly used genomics assays-the 21-gene test (Oncotype DX),^36,37 PAM-50,³⁸ and the 70-gene test (MammaPrint),³⁹ have been validated primarily in European and North American populations, yet breast cancer gene expression profiles may differ across ethnic groups. The RecurIndex (28-gene) genomic risk score was specifically developed for Asian breast cancer populations. Previous studies have demonstrated its ability to effectively stratify locoregional recurrence (LRR) risk in N0 and N1–N2 disease, with 3-year LRR rates <3% in the low-risk group and >50% in the high-risk group, achieving an estimated accuracy of 75%–78%.⁴⁰ However, these findings may not reflect contemporary outcomes with modern systemic therapy, and LRR rates in the current trial are anticipated to be substantially lower due to advances in systemic treatment and more refined risk stratification. Subsequent refinement reduced the gene panel from 34 to 18, using a threshold of 44 points, which improved accuracy to 93% (sensitivity 87%, specificity 94%).²⁰ We acknowledge that the RecurIndex assay, while developed in Asian populations and supported by institutional and regional cohort data, has not yet undergone large-scale, independent external validation. Therefore, its role in this trial is exploratory and hypothesis-generating. Future validation in multi-institutional, prospective cohorts will be necessary to establish its clinical utility for guiding radiotherapy decisions.

Prior to the present study, our research has initiated two randomized controlled trials to guide RNI field for pN1 breast cancer after ALND by using clinical model combined with RecurIndex.^23,41 Until September 30 2024, the IMNI PRECISION trial has enrolled a total of 253 clinical “high-risk” pN1 BC patients, with 141 clinical-genomic high-risk patients (55.73%), and 112 clinical high-risk, but genomic low risk (44.26%). The preliminary results indicate that using clinical-genomic model could further individualize the local recurrence risk of BC patients. Based on these previous works, our research team conducts this prospective RNI PRECISION trial as a proof-of-concept study. We aim to investigate whether individualized RNI based on a combined clinical-genomic model might reduce 2-year BCRL in comparison to SLNB+ALND cohort.

The current trial has several potential limitations. First of all, the primary outcome is 2-year BRCL, not survival outcomes. Although BC patients with lymphoedema experience a significantly lower QoL, comparable survival outcomes from individualized RNI are also important for this patient population. Therefore, a non-inferior randomized controlled trial is still needed to confirm the efficacy of individualized RNI in BC patients with limited SLN metastasis. Secondly, this is a single institute, open-label, non-randomized cohort trial, which is an inherent limitation. While we employed rigorous statistical methods (IPTW/PSM) to mitigate confounding by indication, unmeasured factors may influence outcomes. The study is therefore best interpreted as a comparison of real-world management strategies rather than a definitive assessment of causality. In addition, the generalizability of our clinical-genomic algorithm and the proposed dose constraints requires validation in multi-institutional settings. Thirdly, while the sample size is based on a substantial absolute risk reduction observed in prior studies and supported by dosimetric rationale, the design cannot isolate the independent effect of RNI volume reduction, as surgery and radiation strategy are altered simultaneously. Any observed difference in BCRL between cohorts should be interpreted as the combined effect of de-escalated surgery and personalized RNI, rather than attributable solely to radiotherapy modifications. Finally, we acknowledge that more sensitive objective assessment tools, such as optoelectronic perometry, are increasingly utilized in contemporary lymphedema trials. As perometry is not routinely integrated into our clinical workflow, we employed a standardized circumferential measurement method, which represents a well-established, reproducible, and clinically accepted alternative.³⁰ And perometry will be recommended for future multi-center validation studies.

Conclusion

In the current report, we present the protocol of the RNI PRECISION trial, which aims to investigate whether individualized RNI according to recurrence risk by using a combined clinical-genomic model for SLNB cohort could lead to a reduction in 2-year BCRL in comparison to SLNB+ALND cohort. This proof-of-concept study is positioned to generate high-quality preliminary evidence on the feasibility, safety, and potential benefit of personalizing RNI, with the goal of refining the decision algorithm for future definitive trials.

Supplemental Material

Supplemental Material - RNI Precision Trial Protocol: Optimization of Regional Node Irradiation for Sentinel Lymph Node-positive Breast Cancer Omitting Axillary Dissection Based on Clinical and Genomic Risk Assessment: A Prospective Clinical Trial

Supplemental Material for RNI Precision Trial Protocol: Optimization of Regional Node Irradiation for Sentinel Lymph Node-positive Breast Cancer Omitting Axillary Dissection Based on Clinical and Genomic Risk Assessment: A Prospective Clinical Trial by Wei-Xiang Qi, Lu Cao, Siyue Zheng, Shuyan Li, Feifei Xu, Cheng Xu, Rong Cai, Gang Cai, Jiayi Chen in Technology in Cancer Research & Treatment

Supplemental Material

Supplemental Material - RNI Precision Trial Protocol: Optimization of Regional Node Irradiation for Sentinel Lymph Node-positive Breast Cancer Omitting Axillary Dissection Based on Clinical and Genomic Risk Assessment: A Prospective Clinical Trial

Supplemental Material

Supplemental Material - RNI Precision Trial Protocol: Optimization of Regional Node Irradiation for Sentinel Lymph Node-positive Breast Cancer Omitting Axillary Dissection Based on Clinical and Genomic Risk Assessment: A Prospective Clinical Trial

Footnotes

ORCID iDs

Wei-Xiang Qi

Jiayi Chen

Ethical Considerations

This study was approved by the Ethics Committee of Ruijin hospital, Shanghai Jiao Tong University School of Medicine (RJ2024-50).

Consent to Participate

Informed consent to participate was obtained from all of the participants in the study. The study is registered in (NCT04320979). Written informed consent will be obtained from all participants included in the study prior to randomization.

Author Contributions

Principal investigator: J.C. G.C. and W.X.Q.

Drafting of the protocol manuscript: W.X.Q. and G.C.

Conceptualization: J.C. G.C. and W.X.Q.

Project administration: C.X. F.X., S.L. and L.C.

data analysis, acquisition, and interpretation: S.Z., S.W., W.X.Q., G.C., J.C.

manuscript preparation: W.X.Q., C.X., G.C., J.C., F.X., L.C., R.C., S.W. S., Z. S.L.

Final approval of manuscript: all authors.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by Clinical Research Special Project of Shanghai Municipal Health Commission Health Industry (202340226), Shanghai Science and Technology Innovation Action Plan Medical Innovation Research Project (23Y11904700) and Shanghai Key Laboratory of Proton therapy (23dz2261000). The funding source has no role in study design, data collection, analysis, interpretation, the writing of the manuscript, or the decision to submit the current study.

Declaration of Conflicting Interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data Availability Statement

Data sharing is not applicable to this article as the current study is still open for inclusion of patients.*

Trial Registration

NCT06583655 at 2024-09-01 prospectively registered.

Date of Registration

September 01, 2024. Status: Recruiting.

Supplemental Material

Supplemental material for this article is available online.

Appendix

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