Osimertinib‐induced <scp>BRAF</scp> mutation in a single metastatic lesion among multiple pulmonary lesions in a case of lung cancer with <scp>EGFR</scp> exon 19 deletion

One of the resistant mechanisms of EGFR-TKIs is BRAF V600E mutation. Herein, we present the case of a 54-year-old Japanese female who underwent a right middle lobectomy for pathological stage IIB lung adenocarcinoma. One year and nine months after the surgery, she developed multiple intrapulmonary metastases. Osimertinib was administered due to EGFR exon 19 deletion. Although all intrapulmonary metastases had shrunk, the nodule at the superior segment of left lung enlarged after postoperative 4 years. The tumour was resected and BRAF V600E mutation and exon 19 deletion were detected. Three months after treatment with dabrafenib and trametinib instead of osimertinib, the remaining intrapulmonary metastases increased again. The continued growth of the metastatic foci even after EGFR-TKI may indicate an acquired resistance. Thus, a repeat biopsy will aid in confirming the new gene expression. It should have been necessary to administer an additional dose of dabrafenib and trametinib without discontinuing osimertinib.