Combined Small and Squamous Transformation in EGFR-mutated Lung Adenocarcinoma.

Histologic transformation has been described as an acquired mechanism of resistance to epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs). We herein report the case of a woman with stage IV lung adenocarcinoma harboring EGFR exon 19 deletions who was initially treated with EGFR-TKIs; several cytotoxic chemotherapeutic regimens were used when resistance developed. A lymph node re-biopsy revealed histologic transformation of the tumor to combined small-cell lung cancer and squamous cell carcinoma with retained EGFR exon 19 deletions. Following sequential chemotherapy appropriate for transformed histology, a clinical response was achieved.