BACKGROUND
Various resistance mechanisms of the epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKI) have been reported, and approximately half of the cases show a T790M point mutation as resistance to EGFR-TKI. In addition, 3-14% of cases of non-small cell lung cancer transform into small cell lung carcinoma (SCLC) during treatment. However, there are few reported cases in which 2 mechanisms of resistance have been observed simultaneously. This report describes a 66-year-old man with initial presentation of stage IIA right-sided lung adenocarcinoma with EGFR gene exon 21 L858R mutation and 3 years of stable disease. During treatment with erlotinib, the patient developed SCLC and adenocarcinoma with EGFR exon 21 L858R and exon 20 T790M mutation.
CASE REPORT
A 66-year-old man underwent right pneumonectomy plus nodal dissection 2a for right hilar lung cancer and was diagnosed with an EGFR exon21 L858R mutated lung adenocarcinoma. Three years later, pleural dissemination was observed in the right chest wall. Although erlotinib was continued for 52 months, new metastases to the right ribs were detected. Chest wall tumor resection was performed. Based on the World Health Organization classification, the patient was diagnosed with combined SCLC, with EGFR exon21 L858R and exon20 T790M mutation. The patient received 4 cycles of carboplatin plus etoposide, 14 cycles of amrubicin, and 2 cycles of irinotecan. Chemotherapy continued for 25 months.
CONCLUSIONS
Long-term survival was achieved by chemotherapy after transformation. Since EGFR mutation-positive lung cancer shows a variety of acquired resistances, it is important to consider the treatment strategy of performing re-biopsy.

Keywords: Biopsy; Small Cell Lung Carcinoma; Adenocarcinoma of Lung; EGFR Tyrosine Kinase Inhibitor 324674