Survival outcomes and treatment patterns by tumor subtype (TS) in stage III inflammatory and non-inflammatory breast cancer (BC).

607 Background: Inflammatory BC (IBC) is an aggressive form of BC for which trimodality therapy (TMT) is standard. We examined TMT use and compared BC-specific survival (BCSS) and overall survival (OS) between IBC and non-IBC within TS. We also examined survival changes over time. Methods: We evaluated patients (pts) with stage III BC diagnosed 2010 - 2020, using Surveillance, Epidemiology and End Results data. The cohort was categorized as IBC (T4d), and non-IBC. TMT included neoadjuvant systemic therapy (NAST), mastectomy, dissection of ≥6 axillary nodes, and adjuvant radiation (RT). We evaluated BCSS and OS between IBC and non-IBC stratified by TS. Comparisons were made in stratified multivariable Cox models providing adjusted hazard ratios (aHzR) with 95% confidence intervals (CI). Results: We included 3,227 IBC and 44,475 non-IBC pts, with a median follow up of 70 and 68 months, respectively. Median age was 57 years for both. As compared with non-IBC, IBC pts were more often White, single, and had more grade III and less HR+/HER2- (all p<0.001). Individual treatments in IBC vs non-IBC included: mastectomy (73.5% vs 68.1%, p<0.001); dissection of ≥6 axillary nodes (54.9% vs 71.6%, p<0.001); RT (58.4% vs 60%, p=0.07), chemotherapy (87.1% vs 78.7%, p<0.001), NAST (58.4% vs 32.7%, p<0.001); respectively. Overall rate of TMT for IBC was 31.4% and decreased from 33.9% in 2010 to 24.2% in 2020 (p for trend = 0.014). Stage III IBC pts had worse BCSS than non-IBC in each TS (table). Pts with T4 tumors had similar differences but with smaller magnitude (table). We observed improvements in BCSS between 2010-2020 for stage III non-IBC (aHzR=0.98 per year, p<0.001), non-IBC T4 only (aHzR=0.97 per year, p<0.001), and stage III non-IBC treated with TMT (aHzR=0.97 per year, p=0.007). In contrast, there were no improvements in BCSS between 2010-2020 for IBC pts overall (aHzR=1.02 per year, p=0.12) or IBC pts treated with TMT (aHzR=1.03 per year, p=0.25). OS showed similar results to those from BCSS across all analyses. Conclusions: Stage III IBC has worse BCSS and OS than stage III non-IBC within each TS. The rate of TMT for IBC is low and decreased significantly over time, though the completeness of treatment data is a limitation. BCSS and OS did not improve over time for IBC pts, even when treated with TMT; whereas BCSS and OS improved for non-IBC. Table: see text