Abstract PS11-01: Brain metastases in metastatic breast cancer: prevalence per line of treatment and cumulative incidence in a cohort of 18075 real-world patients

Abstract BACKGROUND Historic data suggest that incidence of brain metastasis (BM) is relatively distributed over time in HER2-positive (HER2+) breast cancer (BC), occurs early in triple negative BC (TNBC), and occurs late in hormone receptor-positive (HR+) metastatic BC (mBC). However, the timing of BM relative to line of therapy has not been well described. We describe the prevalence per line of therapy and cumulative incidence of BM in a large real-world cohort of patients (pts) with mBC, by BC subtype and line of therapy. METHODS This analysis used data from the longitudinal US Flatiron Health de-identified database, which provides unstructured and structured electronic health record-derived data from 2.6 million pts with cancer in ~800 unique sites of care. Eligible pts had initiated a first line of treatment (index data) for mBC up to March 1, 2021, to allow for ≥2 years potential follow-up time. Baseline characteristics were assessed, and pts categorized by HER2 and HR status: 1) HR+, HER2+; 2) HR+, HER2–, 3) HR–, HER2+ and 4) TNBC (i.e., negative for HR and HER2). For the last two categories, HER2-low subsets were defined based on immunohistochemistry results (1+, 2+, or equivocal). Lines of therapy were derived using both treatment regimens and progression data. The primary outcome was the first diagnosis of BM. The prevalence of BM was evaluated by subtype, at the index date, and by the line of therapy. The cumulative incidence function (CIF) of BM was used to estimate the risk of BM in pts free of BM at the index date, and death was treated as a competing event. RESULTS Overall, 18075 pts were included (HR+, HER2+: 3062 pts 16.9%; HR–, HER2+: 902 pts 5.0%; HR+, HER2–: 12331 pts 68.2% HR+, HER2-low: 7062 (39.1%); TNBC: 1780 pts 9.8% HR–, HER2-low: 725 pts (4.0%)). Median age at the index date was 64 years (interquartile range: 54, 73) and 10271pts (56.8%) had visceral metastasis. In total, 5951 pts (32.9%) had de novo disease, 12090 (66.9%) had recurrent disease, and 34 (0.2%) did not have available data for mBC type. The table shows the number of pts by line of therapy, prevalence of BM, and cumulative incidence of BM from the index date. Of the included pts, 1306 (7.2%) had a BM at the index date; the CIF was run on the remaining 16973 pts. Overall, 2248 pts (13.2%) had an incident BM event during follow-up (HR+, HER2+: 578 events; HR–, HER2+: 237 events; HR+, HER2–: 1119 events HR+, HER2-low: 619 events; TNBC: 314 events HR–, HER2-low: 124 events), 9314 had a competing event, and 5411 were censored. By fourth-line therapy, the prevalence of BM was 26.1% in HR+, HER2+; 37.1% in HR–, HER2+; 24.7% in TNBC (HR–, HER2-low: 27.9%); but remained low at 7.2% in HR+, HER2– (HR+, HER2-low: 9.4%). The cumulative incidence of BM at 60 months was 23% in HR+, HER2+, 34% in HR–, HER2+, 10% in HR+, HER2–, and, 22% in TNBC. Overall, the HER2-low subsets had BM prevalence and cumulative incidence that were very close to those that were HER2–. CONCLUSIONS This large analysis of real-world data demonstrates that the prevalence of BM across pts with mBC differs by tumor subtype and by line of therapy. Conversely, HER2-low status may have limited impact. The cumulative incidence of BM was highest in the HR–, HER2+ and TNBC subgroups, and lowest in the HR+, HER2– subgroup; despite this, due to the high proportion of HER2–, HR+ mBC cases, the HR+, HER2– group represented the largest number of BM events. These data emphasize the need for clinical and biologic predictors of BM and for strategies to prevent their onset, and provide information on the impact of BM inclusion and exclusion criteria in clinical trials for pts with mBC. Number of patients by line of therapy, prevalence of BM, and cumulative incidence of BM from the index date BM, brain metastasis; HR, hormone receptor, pts, patients; TNBC, triple negative breast cancer. Citation Format: Sarah Sammons, Jose Leone, Thibaut Sanglier, Peter Lambert, Filippo Montemurro, Raf Poppe, Eleonora Restuccia, Sara Tolaney, Nancy Lin. Brain metastases in metastatic breast cancer: prevalence per line of treatment and cumulative incidence in a cohort of 18075 real-world patients abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS11-01.