EGFR and cancer: Mutations in cancer-associated genes were analysed using next-generation sequencing of circulating tumour DNA samples collected before osimertinib treatment and on detection of PD.<h4>Findings</h4>Between May 2019 and January 2021, 188 patients were enrolled, of whom 125 (66%) were women and 96 (51%) had EGFR exon 19 deletion mutations.