Abstract PS3-07-03: Evaluating Nodal Burden in T1a-T2 HR+HER2- Breast Cancer: Implications for SLNB Omission in Patients ≥50 Years

Abstract Purpose: The recent INSEMA and SOUND randomized trials support sentinel lymph node biopsy (SLNB) omission in postmenopausal patients ≥50 years with clinically node-negative T1-T2 hormone receptor-positive (HR+) HER2- breast cancer and negative axillary ultrasound. We used real-world data to evaluate pathologic nodal positivity rates by tumor size for HR+HER2- breast cancer to assess the appropriateness of SLNB omission. Methods: We performed a population-based retrospective cohort study using ICES administrative databases. Patients diagnosed with T1-T2 HR+HER2- invasive breast cancer from 2000-2019 in Ontario, Canada were identified through the Ontario Cancer Registry. Demographic, tumor, and treatment data were linked. Nodal positivity rates across T1a-T2 were calculated for the full cohort and stratified by age group (50, 50-69, ≥70). Multivariable logistic regression adjusting for age, T stage, and hormone receptor subtype was performed using SAS®. Results: There were 30,969 HR+HER2- tumors, including 4895 (15.8%) patients 50 years; 16,987 (54.9%) between 50-69 years; and 9087 (29.3%) ≥70 years. Among patients aged 50, 1944 (39.7%) were node-positive, most of whom (1590, 81.8%) had N1 disease, followed by 278 (14.3%) N2 and 76 (3.9%) N3. By T stage, there were 21 (1.1%) T1a, 75 (3.9%) T1b, 582 (29.9%) T1c, and 1266 (65.1%) T2 tumors with nodal involvement. The majority of N2 and N3 disease occurred in T2 tumors 212 (76.3%) N2; 60 (78.9%) N3, followed by T1c tumors 60 (21.6%) N2; 15 (19.7%) N3. Among patients aged 50-69, 4756 (28.0%) were node-positive, of whom there were 3905 (82.1%) N1, 634 (13.3%) N2, and 217 (4.6%) N3 disease. By T stage, there were 45 (0.9%) T1a, 306 (6.4%) T1b, 1775 (37.3%) T1c, and 2630 (55.3%) T2 tumors with nodal involvement. The majority of N2 and N3 disease occurred in T2 tumors 463 (73.0%) N2; 168 (77.4%) N3, followed by T1c tumors 151 (23.8%) N2; 39 (18.0%) N3. Among patients aged ≥70, 2295 (25.3%) were node-positive, of whom there were 1890 (82.4%) N1, 298 (13.0%) N2, and 107 (4.7%) N3 disease. By T stage, there were 10 (0.4%) T1a, 100 (4.4%) T1b, 755 (32.9%) T1c, and 1430 (62.3%) T2 tumors with nodal involvement. The majority of N2 and N3 disease occurred in T2 tumors 216 (72.5%) N2; 92 (86.0%) N3, followed by T1c tumors 75 (25.2%) N2; 13 (12.1%) N3. In our cohort, SLNB was omitted in 288 (3.2%) patients over age 70. On multivariable logistic regression, tumor size was the strongest predictor of nodal positivity. Compared to T1a tumors, the odds of nodal involvement were significantly higher for T1b (OR 1.9, 95% CI 1.5-2.5, P0.01), T1c (OR 6.0, 95% CI 4.8-7.6, P0.01), and T2 tumors (OR 17.0, 95% CI 13.4-21.5, P0.01). Age was also inversely associated with nodal positivity. Compared to patients 50, the odds of nodal involvement were significantly lower for those ≥50 years (OR 0.7, 95% CI 0.7-0.8, P0.01) and those ≥70 years (OR 0.6, 95% CI 0.5-0.6, P0.01). Hormone receptor subtype was also associated with nodal positivity. Compared to ER+PR+ tumors, the odds of nodal involvement were significantly lower for ER+PR- (OR 0.9, 95% CI 0.8-1.0, P=0.02) and ER-PR+ (OR 0.7, 95% CI 0.5-0.9, P=0.01). Conclusion: Rates of nodal positivity are not insignificant for early-stage breast cancer. In women over the age of 50 where SLNB omission may be considered, rates of nodal positivity increased from 5% for pT1a lesions to 25% for pT1c lesions. Most of the nodal burden was N1 disease. This suggests that SLNB omission would not have impacted systemic therapies, as most of these patients would receive genomic assay testing. However, radiotherapy decisions would be impacted as the identification of nodal disease is an indication for locoregional radiation. These findings need to be considered in the era of surgical and radiotherapy de-escalation. Citation Format: Y. R. Lee, D. W. Lim. Evaluating Nodal Burden in T1a-T2 HR+HER2- Breast Cancer: Implications for SLNB Omission in Patients ≥50 Years abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS3-07-03.