Recent advances in breast cancer management might make the use of postmastectomy radiotherapy (PMRT) redundant in the treatment of pT1/T2N1 patients. We investigated the impact of PMRT on disease-free survival (DFS) in these patients who have a low risk of locoregional recurrence (LRR) after contemporary multidisciplinary management. Between 1998 and 2011, 1123 patients underwent upfront surgery for pathologically diagnosed pT1/T2N1 breast cancer, at a single institution. A retrospective review was performed on 692 patients who had a mastectomy with axillary lymph node (LN) clearance. Most patients received adjuvant systemic chemotherapy and/or endocrine therapy. PMRT was administered to 17.8% of the patients. The median follow-up time was 98 months. The entire cohort was divided into 2 groups, the early-era (1998-2003) and late-era (2004-2011) cohorts. Grouping was based on the use of modern therapies since 2004 including sentinel LN (SLN) biopsy, anthracycline/taxane-based chemotherapy, and aromatase inhibitors. Late-era patients had a significantly lower 5-year LRR compared with early-era patients (3.2% vs 10.3%, respectively; P<0.001). In late-era patients, although PMRT did not significantly reduce the 5-year LRR rate (1% vs 3.8%, respectively), it did improve the 5-year DFS rate (96.1% vs 87.5%, respectively). After controlling for all clinicopathological variables, PMRT was independently associated with improved DFS. In subgroup analysis, depending on the presence of micro- or macrometastasis in the axillary nodes, the benefit of PMRT was most apparent in patients with macrometastasis (hazard ratio, 0.19). In the late-era cohort with no PMRT, the 3-year distant metastasis risk increased according to LN tumor burden (0%, 5.2%, and 9.8% in micrometastasis, SLN macrometastasis, and non-SLN macrometastasis, respectively). Advanced surgical and systemic therapies might not negate the benefit of PMRT in recently diagnosed pN1 patients who have a very low risk for LRR. Our data indicate that the overall recurrence risk combined with the LRR should be considered for an indication of PMRT, and raises the question of whether the receipt of PMRT would improve outcome in patients with micrometastasis.
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