Patterns of progression in oligometastatic hormone receptor positive breast cancer after first-line systemic treatment at the metastatic stage.

e13054 Background: In recent years, thanks to large, consecutive and retrospective series, knowledge of oligometastatic breast cancer (OMBC) has improved. However, the underlying pathophysiological mechanisms and patterns of progression of OMBC remain largely unknown. The aim of this study is to determine whether the limited capacity of OMBC to metastasize persisted in patients receiving first-line systemic treatment at the OMBC stage. To obtain a biologically homogeneous cohort, the analysis focused on hormone receptor positive (HR+) OMBC. Methods: We retrospectively reviewed all patients consecutively diagnosed and treated from 2012 to 2020 at our institution for synchronous and metachronous HR+ OMBC. Included were HR+ MBC patients with up to five metastases at diagnosis with no other criteria. Clinical and biological characteristics at diagnosis of first metastases, types of treatment (local ablative treatment of all sites (LAT) vs palliative treatment), outcomes and patterns of progression after a first line systemic treatment at the oligometastatic stage were recorded. Progression-free survival (PFS) and overall survival (OS) rates with a 95% confidence interval were estimated using the Kaplan Meier method. Results: Out of 1,686 patients treated for MBC at our institution between 2012 and 2020, 11.0% (186/1,686) met inclusion criteria. Among these 186 HR+ OMBC patients, 62.2% had SBR grade I/II HR+, and 19.3% had HER2+ OMBC. 93.5% patients had only one organ invaded, and 88.2% patients had one to three metastases at diagnosis. 66.7% had bone metastases, 60.2% had bone-only metastases, 18.3% had visceral (lung or liver) metastases, 14.5% had lymph node metastases and 6.5% had brain metastases. 35.5% of patients were treated with a curative intent including systemic treatment plus LAT. After a median follow-up of 73.4 months (95%CI= 69.3-76.8), median PFS and OS were respectively 31.6 months (95%CI= 23.7-37.4) and 90.9 months (95%CI= 70.0-109.9). Five years PFS and OS were respectively 32.6 % (95%CI= 25.5-39.9) and 66.3% (95%CI= 58.4-73.1). During this period, 64.0% (119/186) of HR+ OMBC patients presented disease progression. Among the latter, 74.1% showed spread with no more than five metastases (oligoprogression), and 82.6% in only one organ. 27.7% (33/119) of patients were able to benefit from LAT. Conclusions: HR+ OMBC incidence is 11%. Among patients with progressive HR+ OMBC following first-line systemic treatment at the oligometastatic stage, a large majority presented oligoprogression and a substantial proportion were able to be treated by local ablative therapy with a curative intent at the time of this progression. The limited capacity of HR+ OMBC to metastasize thus persisted after a first-line systemic treatment. This could be an argument in favor of physiopathological mechanisms specific to oligometastatic disease.