Pulmonary micropapillary‐type adenosquamous carcinoma sharing epidermal growth factor receptor mutation in adenocarcinoma and squamous cell carcinoma

Adenosquamous lung carcinoma (AdSqLC) has a worse prognosis than adenocarcinoma (ADC) or squamous cell carcinoma (SQCC). Micropapillary pattern in lung ADC is an additional poor prognostic factor. We describe a rare case of AdSqLC with epidermal growth factor receptor (EGFR) mutation in both the micropapillary‐ADC and SQCC components, showing long‐term response to gefitinib. A 60‐year‐old woman underwent right lower lobectomy for primary lung cancer. Histopathological examination demonstrated adenosquamous carcinoma comprising micropapillary‐ADC and moderately differentiated SQCC. EGFR exon 19 deletions mutation was detected in both the ADC and SQCC components. Gefitinib was administered for multiple metastatic recurrences on bilateral lung, resulting in remarkable shrinkage of visible lesions. The efficacy of gefitinib lasted for 31 months after the induction. AdSqLCs harbouring the EGFR mutation in both the ADC and SQCC components may well benefit from EGFR tyrosine kinase inhibitors, especially when they contain micropapillary‐ADC component that correlates with frequent EGFR mutations.