Impact of Post-Mastectomy Radiation Therapy in Women With T1-2N1M0 Breast Cancer With Sentinel Lymph Node Biopsy Alone.

PURPOSE/OBJECTIVE(S) To report outcomes for women with pT1-2N1M0 breast cancer undergoing mastectomy and sentinel lymph node biopsy (SLNB) without completion axillary dissection, and to investigate the effect of post-mastectomy radiation therapy (PMRT). MATERIALS/METHODS Data from 5 North American institutions were pooled to include women with pT1-2N1M0 breast cancer from 1997-2015 who underwent mastectomy with a positive SLNB and ≤ 5 lymph nodes (LNs) resected. All patients were clinically N0 and none received neoadjuvant chemotherapy. Competing-risks regression was performed to identify factors associated with overall recurrence (OR), inclusive of locoregional (LR) and distant recurrence (DM). RESULTS Among 871 patients included, median age at diagnosis was 57 years, 92% were ER/PR-positive, 85% HER2-negative, and 60% received adjuvant chemotherapy. The 500 (57%) patients who received PMRT were significantly younger, had larger tumors, more positive LN, lymphovascular invasion (LVI), extracapsular extension (ECE), HER2-positive disease, and more frequently received adjuvant chemotherapy (all P < 0.05) compared to those not receiving PMRT. Median number of lymph nodes sampled in each group was 3 (range: 1-5). At a median follow up of 5.2 years, the 5-year cumulative incidence rates of LR, DM, OR, and breast cancer mortality (BCM) were 2%, 4%, 5%, and 3%, respectively. The 5-year rates of disease-free survival (DFS) and overall survival (OS) were 87% and 91%. LR was the only endpoint with a statistically significant difference between those receiving PMRT (0.4%) as compared to those without PMRT (3.9%), P < 0.001. On univariable competing risks regression, tumor size, nodal ratio, grade, LVI, and ER/PR status were significantly associated with OR and were included in multivariable analysis. On multivariable competing risks regression, larger tumor size (HR 1.47, 95% CI 1.19-1.81, P < 0.001), grade III (HR 1.81, 95% CI 1.07-3.05, P = 0.03), higher nodal ratio (HR 2.86, 95% CI 1.01-7.69, P = 0.049), and LVI (HR 2.04, 95% CI 1.27-3.28, P = 0.003) were significantly associated with increased risk of OR. PMRT was associated with decreased risk of OR (HR 0.60, 95% CI 0.26-1.00, P = 0.05), though this effect did not reach statistical significance. CONCLUSION In selected patients undergoing mastectomy and SLNB without completion axillary dissection for pT1-2N1 breast cancer, larger tumor size, higher grade, increased nodal ratio and the presence of LVI are associated with increased risk of overall recurrence. Further studies are needed to identify which subset of patients may benefit most from PMRT.