Abstract PS2-03-13: Breast surgery and systemic treatment continuation for patients with de novo metastatic breast cancer and locoregional oligoprogression: a cohort study

Abstract Background: In patients with de novo metastatic breast cancer (mBC), two randomized clinical trials demonstrated that upfront breast surgery does not improve clinical outcomes. However, the role of breast surgery in patients with a locoregional oligoprogression (OPD), in the breast or locoregional lymph nodes, who previously had a response to systemic treatment is unknown. This approach may allow to maintain the same systemic treatment and prolong the time to treatment failure. Methods: We conducted a prospective-retrospective, single-center, cohort study including consecutive patients with de novo mBC, who experienced an OPD in the breast and/or locoregional lymph nodes (with other sites maintaining the best response), underwent breast surgery, and continued the same systemic treatment after surgery, according to the indication of the multidisciplinary tumor board at the European Institute of Oncology (Milan, Italy) from January 2007 to March 2025. The primary endpoint was post-surgery progression-free survival (pS-PFS), defined as the time from the first radiographic evidence of locoregional OPD to subsequent progression or death. Results: 59 patients were included: 28 (47%) had Hormone Receptor (HR)+/HER2- disease, 30 (51%) HER2+, and 1 (2%) triple-negative. When OPD occurred, the median line of treatment was 1 (IQR: 1-2), and the median pre-OPD PFS was 15.2 months (95% CI, 12.0-18.4). In the HR+/HER2- subgroup 22/28 (79%) patients were receiving an endocrine therapy-based therapy, in the HER2+ subgroup 19/30 (63%) were on trastuzumab/pertuzumab maintenance. The median time between OPD and surgery was 1.5 months (95% CI, 1.3-1.7). 12 (20%) patients underwent lumpectomy, 45 (75%) mastectomy, 5 (9%) sentinel lymph node biopsy, and 27 (46%) lymph node dissection. After a median follow-up of 54.1 months (95% CI 44.2-64.0), 38 pS-PFS events occurred in all patients, 21 in the HR+/HER2- subgroup, and 15 in the HER2+ subgroup. The median pS-PFSs were 15.0 months (95% CI, 3.7-26.3) in all patients, 9.0 months (95% CI, 4.9-13.1) in the HR+/HER2- subgroup, and 24.2 months (95% CI, 8.2-40.3) in the HER2+ subgroup. Among patients with a pre-OPD PFS ≥12 months, the median pS-PFSs was 21.4 months (95% CI, 10.2-32.6), as compared with 10.5 months (95% CI, 4.4-16.6) among those with a pre-OPD PFS <12 months (log-rank test: p=.027). Conclusions: This study suggests that selected patients with de novo mBC and locoregional OPD can benefit from breast surgery while maintaining the same systemic treatment, especially in case of HER2+ disease or a longer a pre-OPD PFS. Citation Format: N. Bianco, C. Valenza, M. Milano, D. Trapani, S. Di Bella, A. Sciandivasci, I. Minchella, P. Veronesi, V. Galimberti, M. Intra, C. Sangalli, E. Munzone, G. Curigliano, M. Colleoni. Breast surgery and systemic treatment continuation for patients with de novo metastatic breast cancer and locoregional oligoprogression: a cohort study [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS2-03-13.