221P Incidence and survival after brain metastases among patients with stage I-III breast cancer: A population-based study

Patients with HER2-positive (HER2+) and triple-negative (TNBC) metastatic breast cancer (BC) have a high propensity to develop brain metastases (BrM), but BrM incidence among patients who present with early-stage BC has not been well studied. In this population-based cohort study, we used linked administrative databases in Ontario, Canada to identify adult patients diagnosed with stage I-III BC between 2009-2021. Cumulative incidence of surgery and/or radiotherapy for BrM (surrogate for BrM) accounting for competing risk of death, time from primary BC diagnosis to local BrM therapy, and survival time from BrM diagnosis date as stratified by stage and subtype were analyzed using SAS. Of 91, 517 BC patients, 1, 858 (2%) received local treatment for BrM. Median IQR age at BrM diagnosis was 52.5 45-62 years and initial stage at BC presentation was as follows: stage I (n=272; 15%), stage II (n=767, 41%), and stage III (n=819, 44%). With a median follow-up of 6.4 years IQR:3.7-9.6, the incidence proportion of BrM was: 0.6% among patients with stage I; 2.4% with stage II; and 6.9% with stage III BC. Subtypes included 406 (22%) hormone-receptor-positive (HR+)/HER2-negative (HER2-), 161 (9%) HER2+/HR+, 126 (7%) HER2+/HR-, 288 (15%) TNBC; unknown subtype, n=877 (47%). Among stage III patients, the incidence proportion of BrM was 16% (TNBC), 13% (HER2+/HR-), 9% (HER2+/HR+), and 4% (HR+/HER2-); corresponding median time from initial BC diagnosis to local BrM therapy was 1.6, 2.0, 2.9, and 3.7 years, respectively. Among patients with stage I-III BC treated locally for BrM, median (m) OS from date of BrM treatment was 5.8 (2-16) months. Survival by BC subtype was as follows, irrespective of initial stage at diagnosis: HER2+ BC, mOS=13.1 (4.6-28) months; HR+/HER2- BC, mOS=5.1 (1.7-15) months; TNBC, mOS=3.6 (1.5-8.7) months. A high incidence of BrM among patients with stage III BC was identified particularly among those with TNBC or HER2+ BC. The true incidence is likely higher as screening for asymptomatic BrM is not typically performed. In addition, mOS may be over-estimated because untreated patients were not included in this analysis.