OBJECTIVES: Case studies can contribute to understanding when and how non-local real-world evidence can be used for local health technology assessment decision-making. This case study investigates whether overall survival estimates (OS) in HER2+ metastatic breast cancer (mBC) observed in the US are transportable to the UK.

METHODS: We analysed the transportability of OS using data from patients with HER2+ mBC in the US nationwide Flatiron Health electronic health record-derived deidentified database and the ESTHER study, a registry of UK patients with advanced HER2+ breast cancer. We defined OS as the time from initiating the first line of therapy (1L) until death. Using inverse odds of sampling weights, we standardised the US data to match the UK target population from the ESTHER study. The a priori selected factors for standardisation included: age, race/ethnicity, ECOG performance status, hormone-receptor status, de novo/recurrent status, time from diagnosis to treatment initiation, presence of brain metastases, and 1L drug class exposure. We then compared unadjusted and population-adjusted OS estimates from the US to the observed estimates from the UK.

RESULTS: The UK cohort included 248 patients [median age, 57 years (IQR: 48-68)], and the US cohort consisted of 907 patients [median age, 61 years (IQR: 52-70)]. The Tipton generalisability index was 0.83, suggesting a high generalizability from the US to the UK. The median OS in the UK was 51.4 months [95% CI: 43.4, 65.8] and was 50.0 months [95% CI: 43.9, 54.7] before adjustment in the US. After adjustment, OS in the US was 52.9 months [95% CI: 45.0, 61.2].

CONCLUSIONS: In individuals with HER2+ mBC, the overall survival estimates in the US were similar to those in the UK, with or without population adjustment. These findings indicate that US overall survival estimates are a reasonable proxy for overall survival estimates in the UK HER2+ mBC population.