Relative depth of invasion (RDI) index and clinicopathologic predictive factors for identifying high axillary nodal burden in ACOSOG Z0011–eligible breast cancer patients.

e12585 Background: Accurate prediction of non-sentinel lymph node (nSLN) involvement remains a significant challenge in tailoring axillary management strategies for ACOSOG Z0011 eligible patients within the Indian population. Existing international nomograms have shown limited applicability in this demographic due to variations in tumor biology, size, and stage at presentation. We aimed to evaluate the Relative Depth of Invasion (RDI) index, a novel microanatomical parameter of sentinel lymph nodes, along with other clinicopathologic factors, as independent predictors of high axillary nodal burden (≥3 metastatic nodes) in ACOSOG Z0011 eligible breast cancer patients. Methods: A retrospective cohort of 100 breast cancer patients with positive sentinel lymph node biopsy (SLNB) who underwent completion axillary lymph node dissection from February 2021 to February 2023 was analyzed after IRB and Institutional Ethics Committee clearance. Data were retrieved from 368 SLNB procedures, focusing on clinicopathologic variables and nSLN status. The RDI index was defined as the ratio of tumor invasion depth to lymph node diameter along the same axis. Logistic regression analysis was employed to identify predictors of high axillary nodal burden, and Receiver Operating Characteristic (ROC) curve analysis was performed to establish an optimal RDI index cut-off value.Statistical analysis was performed using IBM SPSS version 23. Results: The sentinel node positivity rate was 27.2% (100/368). The cohort had a median age of 54 years, with 62% being postmenopausal. Most patients (84%) had unifocal tumors, with a median size of 2.6 cm, and 91% were grade 2/3. 82% were ER-positive, and 10% were HER2-positive. Among SLN-positive cases, 38% demonstrated nSLN positivity, with 68% classified as having low axillary nodal burden and 32% having high axillary nodal burden. ROC curve analysis identified an RDI cut-off value of 0.46[sensitivity 84.4%; specificity 66.2%; AUC = 0.774]. Logistic regression identified tumor size, perineural invasion (PNI), extranodal extension (ENE), and RDI index as independent predictors of nSLN metastases. Furthermore, PNI, ENE, sentinel lymph node ratio (number of metastatic sentinel nodes/total number of sentinel nodes), and RDI index were statistically significant independent predictors of high axillary nodal burden (p < 0.05) Conclusions: The Relative Depth of Invasion (RDI) index represents a novel , clinically relevant parameter for predicting nSLN metastases and high axillary nodal burden in ACOSOG Z0011 eligible breast cancer patients. Validation in larger, diverse datasets is critical to confirm its predictive utility and facilitate the development of a robust, clinically applicable prediction model to refine axillary surgical de escalation in early breast cancer.