Histologic transformation to small-cell lung cancer following gefitinib and radiotherapy in a patient with pulmonary adenocarcinoma.

Targeted therapies against epidermal growth factor receptor (EGFR) have revolutionized the treatment of a subset of lung adenocarcinomas that have EGFR-activating mutations; however, all patients treated with EGFR tyrosine kinase inhibitors (TKIs) ultimately develop resistance. Histologic transformation of EGFR-mutant adenocarcinoma to small cell lung cancer (SCLC) is a resistance mechanism rarely reported in the literature. We describe the case of a woman with metastatic lung cancer adenocarcinoma with mutated EGFR with an initial response to gefitinib and radiation therapy, who progressed after 18 months due to the development of a resistance mechanism. The new biopsy performed after progression highlighted histologic transformation to SCLC, while maintaining the original EGFR mutation. To better identify patients who progress after TKIs and radiation therapy, it is important to perform tumor rebiopsy and collect data to study mechanisms of acquired EGFR TKI resistance.