Histologic changes from adenocarcinoma transforming into small-cell lung cancer (or squamous-cell carcinoma) after epidermal growth factor receptor (EGFR)-tyrosine kinase inhibitor (TKI) medication is known, but its transformation into sarcomatoid carcinoma is rare. center dot Sarcomatoid transformation from adenocarcinoma may metastasize to unusual sites, which heralds a poor prognosis. center dot The present case is the first report describing a patient with lung adenocarcinoma that had transformed first into squamous-cell carcinoma, then later into sarcomatoid carcinoma after osimertinib treatment. center dot Although lung adenocarcinoma with sarcomatoid transformation is known to show a high level of programmed death ligand 1 (PD-L1) expression, our patient did not show such a level of PD-L1 expression. center dot Repeat biopsy for cell transformation should be considered in patients who after EGFR-TKI treatment experience tumor recurrence, because cell transformations could contribute to acquired resistance to EGFR-TKI therapy.
