A case of lung adenocarcinoma with EGFR exon 19 deletion/insertion mutation (T751_I759delinsS) showing response to Osimertinib.

Epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors are effective for treating EGFR mutation-positive non-small cell lung cancer. However, the diversity of EGFR mutations presents a challenge, as treatment strategies for rare variants remain undefined. A 29-year-old nonsmoking man presented to his primary physician in September 2019 with back pain. Chest computed tomography revealed multiple nodular lesionsin the right lung and pleura, leading to a diagnosis of lung adenocarcinoma originating in the right middle lobe (cT4N1M1a, stage IVA). Initial testing with the CobasⓇ EGFR Mutation Detection Kit v2.0, using real-time polymerase chain reaction (PCR), yielded negative results for EGFR mutations. The patient was subsequently referred to our hospital for further treatment. In January 2020, he began combination therapy with atezolizumab, bevacizumab, carboplatin, and paclitaxel. Additionally, a bronchoscopy was conducted at our hospital to identify potential undetected driver gene mutations. Next-generation sequencing (NGS) analysis, performed as part of a clinical trial, revealed the presence of pT751_I759 delinsS, a rare EGFR exon 19 deletion-insertion mutation variant. The companion diagnostic test confirmed the mutation through re-examination using the peptide nucleic acid-locked nucleic acid PCR-clamp method. After the previous treatment regimen lost efficacy, osimertinib was initiated in April 2021. Tumor shrinkage was observed, and the treatment was sustained for 11 months. This case involved a young patient diagnosed with lung adenocarcinoma. Given the clinical presentation, a driver gene mutation was strongly suspected. NGS identified the rare mutation pT751_I759delinsS, and the findings suggested the potential efficacy of osimertinib for this variant.