Minimal Disease in the Sentinel Lymph Node: How to Best Measure Sentinel Node Micrometastases to Predict Risk of Additional Non-Sentinel Lymph Node Disease

Background. Volume of disease in the sentinel lymph node (SLN) is a significant predictor of additional nodal metastasis. This study assesses incidence of residual non-SLN disease in a large cohort of women with minimal SLN metastases and compares three methods of SLN micro-metastasis volume measurement to determine which best predicts residual disease on completion axillary lymph node dissection (cALND). Methods. A total of 505 patients with invasive breast cancer and minimal SLN metastasis (pN1mi or pN0(i+)) underwent cALND and had complete data. All SLNs were evaluated by three measurement methods for volume of metastasis: (1) method of detection (frozen section, routine hematoxylin and eosin, serial hematoxylin and eosin, immunohistochemistry), (2) American Joint Committee on Cancer's AJCC Cancer Staging Manual, 7th edition, N category, and (3) number of metastatic cells (1-100, 101-999, >= 1000). Multivariable logistic regression models were used to predict the presence of additional non-SLN disease. Results. A total of 251 patients (50%) had pN0(i+) and 254 patients (50%) had pN1mi disease. Twelve percent of those with pN0(i+) and 20% with pN1mi had additional non-SLN disease. On multivariate analyses including eight variables, only lymphovascular invasion (odds ratio >2.2, P <0.01) and volume of nodal metastasis as assessed by any method of measurement (method of detection, AJCC, and cell count) were significantly correlated with additional non-SLN disease (P = 0.04, 0.03, and 0.02, respectively). All three models had similar goodness of fit and discrimination (Akaike information criterion = 442, 442, 441; -2log likelihood = 416, 420, 417; concordance index = 0.680, 0.675, 0.676, respectively). Conclusions. A significant proportion of women with minimal SLN metastases have additional non-SLN disease at cALND. Assessments of SLN volume of disease by three different methods of measurement are equivalent for prediction of additional non-SLN metastases.