OBJECTIVES: It is important to evaluate whether real-world evidence from one country can be reliably used for health technology assessments in another. This study evaluates the transportability of real-world overall survival (OS) estimates from the US to England in de novo metastatic breast cancer (mBC) using nationally representative data from both countries.

METHODS: We used US data from the Flatiron Health EHR-derived, deidentified database and English data from the National Cancer Registration and Analysis Service, a nationwide cancer registry. The cohort included patients diagnosed with de novo mBC between 2015-2021, aged ≥18 at diagnosis, and treated with systemic therapy. We defined OS as time from initial treatment to death, censoring patients at their last recorded activity. We examined unadjusted Kaplan-Meier OS estimates and used matching-adjusted indirect comparison to estimate median OS after balancing aggregate characteristics (age, race/ethnicity, ECOG performance status, molecular subtype, treatment history, diagnosis year).

RESULTS: Comparing the US (n=5814) and English cohorts (n=7770), the median age was 63 versus 61, the percent White was 70% versus 88%, and the percent with ECOG ≤1 was 80% versus 87%. By subtype, the US cohort was 69% HR+/HER2-, 21% HER2+, and 10% triple-negative; the English cohort was 66% HR+/HER2-, 23% HER2+ and 11% triple-negative cases. The unadjusted median OS was 40.3 months (95% CI: 38.0-41.9) for the US cohort and 36.9 months (95% CI: 35.6-38.3) for the English cohort. The predicted median OS was 38.2 months (95% CI: 34.0-45.3) for the English cohort – a difference of 1.3 months between observed and predicted survival.

CONCLUSIONS: These results showed that outcomes for patients with mBC are similar in the US and England after accounting for population characteristics. Further research is needed to evaluate the transportability of real-world evidence across additional diseases and between other countries, along with the suitability of other methods of adjustment.