Management of Bone-Only Progressive Disease in Metastatic Breast Cancer—A Retrospective Single-Center Analysis

Background/Objectives: The optimal management of bone-only progressive disease (PD) in metastatic breast cancer remains unclear for several reasons. Radiologic diagnosis of bone PD is complicated by the lack of standardized response assessment criteria, unspecific morphologic changes of the bone, and flare-up phenomena. Furthermore, bone-only disease and oligoprogression have been associated with favorable prognosis challenging a change of systemic treatment with the consequence of limited treatment options in the future. Additionally, bone-only metastatic disease is frequently excluded from clinical trials resulting in scarce data. This study aimed to assess the therapeutic management and outcome of bone-only PD in metastatic breast cancer patients in a real-world academic setting. Methods: A retrospective analysis of all breast cancer patients with bone metastases (BMs) and at least one event of radiologic evidence of bone-only PD and/or the occurrence of a skeletal-related event (SRE) who were treated at the Department of Obstetrics and Gynecology of the Medical University of Vienna, Austria, between 1 January 2015 and 14 December 2021 was performed. In cases of multiple bone-only PD events in one patient only the first event was considered for analysis. All cases with PD in organs other than the bone were excluded. The primary outcome of the study was to assess therapeutic measures of bone-only PD. Secondary outcomes were the time from bone-only PD to next bone PD (TTF BD) and overall survival (OS; time from bone-only PD to death). Predictors of TTF BD and OS were assessed as exploratory outcomes. Results: Out of a total of 308 breast cancer patients with BMs, 57 had at least one event of bone-only PD. In 59.3% of bone-only PD cases the systemic treatment was continued with a numerically higher rate if multiple metastatic sites were present (71.4% vs. 46.4%). In most bone-only PD events the bone-targeted agent (BTA) was continued (94.5%), independent of the total number of metastatic sites. In 24.1% radiotherapy (RT) was administered with similar rates between patients with bone-only and multiple metastatic sites. The median TTF BD was 6.3 months. In multivariate analysis no predictor for TTF BD could be identified including change of systemic treatment, RT, previous BTA treatment duration, number of previous treatment lines for the metastatic disease, number of metastatic sites and previous or current SRE. Median OS was 21.8 months. Number of previous treatment lines for the metastatic setting was significantly associated with OS with shorter OS in the more advanced disease stage (p-value = 0.0208). Conclusions: Systemic and BTA treatment were continued in the majority of bone-only PD cases. In 24.1% RT was administered. No association between change of systemic therapy and improved oncologic outcome was found. The study’s results are hypothesis-generating in terms of whether change of systemic treatment should be performed restrictively to avoid limited treatment options in the future. Similarly, radiotherapy did not ameliorate prognosis.