Abstract PS3-07-07: Tulip4n+ study: total tumor load as a predictor of ≥4 axillary lymph node metastases in HR+/HER2− breast cancer patients

Abstract Background In early-stage hormone receptor-positive, HER2-negative (HR+/HER2−) breast cancer, the presence of ≥4 metastatic axillary lymph nodes has emerged as a key criterion for selecting candidates for adjuvant abemaciclib, based on data from the MonarchE trial. However, the increasing omission of axillary lymph node dissection (ALND) in clinical practice limits the ability to accurately determine nodal burden. Alternative methods for identifying patients with extensive nodal involvement are therefore needed. The One-Step Nucleic Acid Amplification (OSNA) assay enables intraoperative, quantitative detection of CK19 mRNA in sentinel lymph nodes (SLNs), providing a measure known as total tumor load (TTL). Preliminary studies have suggested a correlation between TTL and nodal involvement, but its utility in predicting ≥4 positive nodes remains to be validated in large cohorts. Objective To evaluate the predictive performance of TTL for identifying ≥4 metastatic axillary lymph nodes in HR+/HER2− early breast cancer and to establish an optimal TTL cut-off to support clinical decision-making, particularly to identify candidates for adjuvant abemaciclib treatment. Methods A retrospective, multicenter analysis was conducted using data from 5,032 patients with HR+/HER2− invasive breast cancer included in six previously published cohorts (2009-2016). All patients underwent SLN biopsy analyzed by OSNA, followed by conventional ALND. Patients with carcinoma in situ, de novo metastatic disease, or who received neoadjuvant therapy were excluded. The association between TTL and the presence of ≥4 positive axillary lymph nodes was analyzed using univariate and multivariate logistic regression models. Additional analyses were performed in histological subgroups, including lobular carcinoma. Results A TTL threshold of ≥1.0 × 105 CK19 mRNA copies demonstrated a positive predictive value (PPV) of 32.5% for identifying patients with ≥4 positive lymph nodes. In the subgroup with lobular carcinoma, the predictive performance improved when TTL was combined with tumor grade and lymphovascular invasion, reaching a PPV of 50%. This multivariate model will be made available as an online calculator to support clinical decision-making. Conclusions The TULIP4N+ study supports the clinical utility of OSNA-based TTL for identifying patients with ≥4 metastatic lymph nodes in HR+/HER2− early breast cancer. In the current era of axillary surgery de-escalation, sentinel lymph node biopsy assessed by OSNA —and especially the TTL value— emerges as a reliable, quantitative tool to guide the selection of patients eligible for adjuvant therapies such as abemaciclib. Citation Format: V. Peg, E. Zamora, I. Sperduti, A. Piñero, J. Fougo. Tulip4n+ study: total tumor load as a predictor of ≥4 axillary lymph node metastases in HR+/HER2− breast cancer patients [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 PS3-07-07.