• Yuji Inagaki, Akihiro Tamiya, Yoshinobu Matsuda, Kouji Azuma, Yuichi Adachi, Takatoshi Enomoto, Shunichi Kouno, Yoshihiko Taniguchi, Nobuhiko Saijo, Kyoichi Okishio, and Shinji Atagi.
• Department of Internal Medicine.
• Medicine (Baltimore). 2020 Oct 16; 99 (42): e22628.

IntroductionThe clinical efficacy of osimertinib for patients with lung adenocarcinoma harboring epidermal growth factor receptor (EGFR) exon 20 insertion mutations is unclear. Few case reports exist on the successful treatment of such tumors with osimertinib. We report a case wherein osimertinib administration had no effect in a patient with EGFR exon 20 insertion-positive lung adenocarcinoma.Patient ConcernsA 48-year-old never-smoking woman was referred to our hospital for chronic cough. Computed tomography (CT) and positron emission tomography-CT revealed a nodule in the right middle lobe, consolidation in the right upper lobe, multiple lymph node metastases, liver metastasis, and multiple bone metastases.DiagnosisOn the basis of further examination using transbronchial lung biopsy, the patient was diagnosed with cT1N3M1 stage IVB lung adenocarcinoma. An EGFR exon 20 insertion, without any additional mutations, was identified.InterventionsDaily oral administration of 80 mg osimertinib was initiated to treat the EGFR exon 20 insertion-positive lung adenocarcinoma.OutcomesAlthough the disease appeared to be stable 2.5 months after the administration of osimertinib, the tumor started to grow 3 months after administration, and carcinoembryonic antigen levels became higher than those before treatment. Thus, osimertinib was discontinued, and treatment with carboplatin as well as pemetrexed and bevacizumab was started, which the patient responded to.ConclusionEGFR exon 20 insertion mutations must be classified in more detail to assess the efficacy of EGFR tyrosine kinase inhibitors. Osimertinib doses that provide favorable therapeutic windows should be considered. Further clinical research is required to clarify the efficacy of osimertinib and other drugs for exon 20 insertion mutations.

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