Locoregional Treatment in De Novo Bone-Only Metastatic Breast Cancer: Prospective, Multi-Institutional Real-World Data, BOMETIN, Protocol MF14-1a.

Introduction: The impact of locoregional treatment (LRT) on survival in de novo bone-only metastatic breast cancer (dnBOMBC) is controversial. This study aims to assess the effect of LRT on survival, utilizing international, prospectively acquired data in this cohort of patients. Materials and Methods: Patients with dnBOMBC were divided into two groups: those receiving systemic therapy only (ST) and those undergoing LRT. Further, patients who received LRT were divided into two subgroups: those who received ST after LRT (LRT+ST group) and those who received ST prior to LRT (ST+LRT group). Factors associated with disease progression, including solitary or multiple bone metastases, were analyzed. Results: There was a total of 744 patients with dnBOMBC treated at each of the participating institutions between 2014 and 2022, with 372 (50%) participants in each arm. Median follow-up was 48 months (32-66, 25-75%). Patients in the LRT group were significantly younger than the ST group [50 (42, 60) vs. 55 (44, 66), p = 0.0001]. There were no significant differences in grade, HER2 status, triple-negative status, receipt of hormonal therapy, or intervention to metastatic sites. During follow-up, 58% (n = 217) of patients in the ST group and 32% (n = 120) of patients in the LRT group died (p < 0.001). Local progression was observed in 20% of the patients in the ST group, whereas 9% progressed in the LRT group (p = 0.0001). Systemic progression occurred more in the ST group; 66% (n = 244) compared to 41% (n = 152) of patients in the LRT group (p < 0.001). The hazard of death was 64% lower in the LRT group than in the ST group (HR: 0.36, 95% CI: 0.29-0.45, p < 0.0001). The burden of metastatic disease differed significantly between the two groups, with a higher rate of solitary bone metastases in the LRT group compared to the ST group (50% vs. 24%, p < 0.001). However, the LRT group had better overall survival (OS) for both solitary (HR: 0.38, 95% Cl: 0.26-0.55) and multiple (HR: 0.38, 95% Cl: 0.29-0.51) bone metastasis patients. Within the LRT group, survival rates were similar whether the breast surgery was performed before or after ST. Multivariate Cox analysis showed that LRT and ER/PR positivity significantly decrease the hazard of death (p < 0.05). Conclusions: Analysis of this large multi-institutional patient cohort provides further evidence that LRT is associated with longer OS and lower locoregional recurrence rates in patients with dnBOMBC. In breast cancer patients with bone-only metastases at presentation, the decision for LRT should be made through a multidisciplinary approach with consideration of surgical therapy at the primary tumor.