Heterogeneity or transformation? A whack‐a‐mole case of EGFR‐mutant lung adenocarcinoma and small cell carcinoma: A case report

Histological transformation from adenocarcinoma to small cell lung cancer (SCLC) occurs ~10% after acquired resistance to epidermal growth factor receptor (EGFR)‐tyrosine kinase inhibitors. Transformed SCLC generally responds well to chemotherapy regimens for SCLC such as platinum plus etoposide. After the response, histological nature and clinical course could be complicated by possible heterogeneity or transformation. Therefore, monitoring rebiospy is desirable to seize its histological nature at that moment. We report a case of EGFR‐mutated adenocarcinoma, where histological transformations from adenocarcinoma and SCLC alternated. In this case, first rebiopsy after gefitinib revealed adenocarcinoma, but second rebiopsy after osimertinib identified SCLC transformation. After failure of platinum plus etoposide, adenocarcinoma‐induced leptomeningeal metastases were controlled by osimertinib reintroduction. Optimal therapies could be provided according to the result of monitoring rebiopsy.