Transformation from acquired EGFR 19del/C797S to EGFR 19del/T790M in an advanced non-small cell lung cancer patient: a case report and literature review.

Third-generation epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs) are the first-line treatment of choice for patients with EGFR-mutant advanced non-small cell lung cancer (NSCLC). However, patients treated with these inhibitors eventually develop resistance. One of the most common mechanisms is the emergence of the EGFR C797S mutation. Whether first-generation EGFR inhibitors (e.g. icotinib or gefitinib) can sustainably control EGFR-sensitive mutations/C797S NSCLC following third-generation EGFR inhibitor treatment remains insufficiently reported. Our case report discusses a female patient with advanced lung adenocarcinoma carrying an EGFR exon 19 E746_A750delELREA mutation who received almonertinib as first-line treatment and developed C797S resistance during therapy. The patient was subsequently treated with a double dose of icotinib for 8 months until disease progression occurred, along with the development of an EGFR exon 20 T790M point mutation and TP53 mutation. This case provides clinical evidence suggesting that first-generation EGFR-TKIs may be an effective treatment strategy for patients with acquired EGFR 19del/C797S resistance following EGFR TKI therapy.