Predictors of Sentinel Lymph Node Metastasis and External Validation of the MSKCC Nomogram in Breast Cancer: A Retrospective Single-Center Cohort Study

Background/Objectives: Predicting sentinel lymph node (SLN) involvement in breast cancer can reduce unnecessary axillary lymph node dissections (ALND) and facilitate clinical decision-making. This study aimed to: (i) identify independent predictors for SLN metastasis, and (ii) assess the predictive performance of the MSKCC nomogram for SLN positivity in early-stage breast cancer patients. Methods: A retrospective cohort study spanning a six-year period was conducted at a tertiary care oncology department in Belgrade, Serbia. The study included 684 consecutive patients with histologically confirmed primary breast carcinoma, without distant metastasis, who underwent surgical treatment with sentinel lymph node biopsy (SLNB). Results: Out of 684 patients, 198 (29%) were SLN-positive. Among the positive cases, 178 (89.9%) had one positive SLN, 19 (9.6%) had two, and 1 (0.5%) had three. Multivariate logistic regression revealed that lymphovascular invasion and tumor sizes >20 mm significantly increased the risk for SLN involvement. Patients with SLN metastasis had significantly higher MSKCC-predicted probabilities compared to SLN-negative patients (52.1 ± 18.5% vs. 33.1 ± 16.9%, p < 0.001). The area under the receiver operating characteristic curve (AUC) for the nomogram was 0.778 (95% CI: 0.738–0.818; p < 0.001). The nomogram demonstrated acceptable discrimination, with an AUC of 0.778 (95% CI: 0.738–0.818). However, calibration-in-the-large was −0.526 (95% CI: −0.709 to −0.342), indicating systematic overestimation of absolute risk. The calibration slope was 1.152 (95% CI: 0.921–1.383), and the Brier score was 0.174. Conclusions: Lymphovascular invasion and primary tumor size >20 mm are the only independent predictors of SLN metastasis. Furthermore, The MSKCC nomogram demonstrated acceptable discrimination but systematically overestimated absolute SLN-metastasis risk. Recalibration and further validation are required before clinical application in this population.