Sequential small cell transformation and T790M mutation in an epidermal growth factor-mutant lung adenocarcinoma: A rare occurrence with significant management implications.

The index case highlights the significance of serial EGFR genotyping along with repeated tissue and/or blood sampling in the prompt detection of genetic and phenotypic resistance mechanisms to EGFR-TKIs. Furthermore, it lends evidence in support of the upfront treatment approaches targeting the heterogeneity of acquired EGFR-TKI resistance mechanisms.