A Case of Lung Cancer Exhibiting Pleoymorphic Carcinoma Transformation Resistance Following Treatment With Osimertinib That Was Successfully Treated Using Local Ablative Treatment.

Various studies have reported resistance mechanisms and treatment methods after epidermal growth factor receptor (EGFR)-tyrosine kinase inhibitor treatment; however, treatment policies have not yet been established, and few cases have reported transformation to pleomorphic carcinoma (PC) as the resistance mechanism. Herein, we report the case of a 66-year-old woman who was diagnosed with Stage 4A lung adenocarcinoma (cT2bN0M1b) through bronchoscopic biopsy. Genetic profiling revealed an EGFR L858R mutation; therefore, osimertinib was administered as the first-line therapy and achieved a partial response. After 46 months of osimertinib treatment, the metastases remained under control; however, the primary tumor enlarged and was therefore resected. Pathological examination confirmed the diagnosis of PC. Genetic testing of the surgical pathology specimen showed that the EGFR mutation L858R was retained, and the patient was considered drug-resistant owing to the histologic transformation to PC. The patient continued osimertinib therapy and had no recurrence at 9 months postoperatively. Transformation to PC following osimertinib administration is rare, and we report this unique case. This study was approved by the Jichi Medical University Saitama Medical Center Ethics Committee (S24-073), and written informed consent was obtained from the patient.