Pathologic nodal burden in TAILOR-RT–eligible breast cancer: An NCDB analysis informing regional therapy de-escalation.

e12616 Background: The optimal extent of axillary management in patients with biologically low-risk, node-positive breast cancer remains uncertain. While OncotypeDX has refined systemic therapy decision-making in estrogen receptor(ER)–positive, HER2-negative disease, its role in guiding locoregional treatment de-escalation is less well-defined. The ongoing TAILOR-RT trial aims to clarify whether regional nodal irradiation (RT) can be safely omitted in select low-risk patients with limited nodal disease. Using a national database, we evaluated the extent of pathologic nodal burden and the association between regional radiotherapy and overall survival in this population with biologically favorable disease and limited nodal burden. Methods: Using the National Cancer Database (NCDB), women age≥40Y with ER+HER2− breast cancer with OncotypeDX Score<18 were identified. Consistent with TAILOR-RT eligibility, the cohort was defined by limited nodal disease (cN1). First, to assess the possibility of greater extent of pathologic nodal burden, we limited our population to only those patients who underwent upfront axillary lymph node dissection (ALND) or initial sentinel lymph node biopsy (SLNB) converted to ALND. Secondarily, we performed a Kaplan-Meier analysis of all patients meeting the above inclusion criteria, who underwent SLNB or ALND, to determine the impact of adjuvant radiotherapy on overall survival in this low-risk population. Results: In total, 3,646 patients met inclusion criteria who had ALND, with 2,745 (75%) having invasive ductal carcinoma and 788 (22%) lobular histology. Median age was 63 years (IQR 54–70). Median lymph nodes examined was 12 (IQR 8–17), with a median of 2 positive nodes (IQR 1–3). On final pathology, 814 patients (22%) had >3 positive nodes, suggesting higher nodal burden than previously estimated. Next, 5101 patients were identified who underwent SLNB or ALND, and had documented receipt (2574, 50.5%) or omission (2527, 49.5%) of regional nodal therapy. Following SLNB and ALND, respectively, 54% (1130/2090) and 48% (1444/3011) received regional radiation. Due to the possibility of selection bias prompting either SLNB or ALND, survival between these groups was analyzed separately to assess the impact of regional radiotherapy, which did not differ (ALND 5YS: NoRT 91.7% vs. RT 93.2% & SLNB 5YS: NoRT 94.7% vs RT 93.1%, p =0.12). Conclusions: While we acknowledge NCDB limitations, including inability to assess breast surgery type, receipt of endocrine therapy, or recurrence, we provide reassuring evidence that most patients meeting TAILOR-RT eligibility did not have markedly higher nodal burden on final pathology. Furthermore, this population may have been adequately treated with axillary surgery alone, without regional radiotherapy. Pending prospective findings, our report further supports axillary therapy de-escalation to reduce morbidity.