The American College of Surgeons Oncology Group (ACOSOG) Z0011 trial demonstrated the safety of omitting axillary lymph node dissection (ALND) in T1-T2cN0 patients with fewer than three positive sentinel nodes (SLNs) undergoing breast-conservation therapy. While microscopic extracapsular extension (mECE) > 2 mm is associated with increased nodal burden, the significance of extranodal tumor deposits (ETDs) in the axillary fat is uncertain.

Consecutive patients with T1-T2cN0 breast cancer undergoing sentinel node biopsy and ALND for SLN metastases from January 2010 to December 2018 were identified. ETDs were defined as intravascular tumor emboli or metastatic deposits in the axillary fat. Clinicopathologic characteristics and nodal burden were compared by ETD status.

Among 1114 patients, 113 (10%) had ETDs: 81 (72%) were intravascular tumor emboli and 32 (28%) were soft tissue deposits. Patients with ETDs had larger tumors (median 2.2 vs. 2.1 cm; p = 0.033) and more often had mECE (83% vs. 44%; p < 0.001). On univariable analysis, presence of ETDs (odds ratio [OR] 9.66, 95% confidence interval [CI] 6.36-14.68), larger tumors (OR 1.47, 95% CI 1.25-1.72), and mECE (OR 10.73, 95% CI 6.86-16.78) were associated with four or more additional positive non-SLNs (NSLNs; all p < 0.001). On multivariable analysis, ETDs remained associated with four or more positive NSLNs (OR 5.67, 95% CI 3.53-9.08; p < 0.001). ETDs were strongly associated with four or more positive NSLNs (OR 7.15, 95% CI 4.04-12.67) among patients with one to two positive SLNs (n = 925).

Among T1-T2cN0 patients with SLN metastases, ETDs are strongly associated with four or more positive NSLNs at ALND. Even among those who may otherwise meet the criteria for omission of ALND, the presence of ETDs in axillary fat warrants consideration of ALND.