Mutations in tyrosine kinase domain of epidermal growth factor receptor (EGFR) are observed in approximately 15% of non-small cell lung cancer adenocarcinoma. Exon 19 deletions or exon 21 L858R mutations are predominant in frequency and show high sensitivity to EGFR tyrosine kinase inhibitors (TKIs). Exon 18 mutations are extremely rare and the delE709_T710insD mutation accounts for only 0.16% of mutations when occurring as a sole mutation. This specific mutation in exon 18 seems to respond to certain EGFR TKIs such as afatinib. However, given the rarity of this mutation, determining the most effective TKI for its treatment remains unclear. We report a 70-year-old woman diagnosed with stage IV-A lung adenocarcinoma harboring EGFR delE709_T710insD mutation treated in first-line with Osimertinib using standard schedule and doses experiencing renal toxicity and disease progression after 9 weeks of treatment.