BACKGROUND There are many complex and rare mutations in the epidermal growth factor receptor (EGFR) gene in non-small cell lung cancer (NSCLC) other than the two classical mutations of L858R and exon 19 deletional mutation. The purpose of this study was to investigate the clinical significance of rare and complex mutations, and the efficacy of EGFR tyrosine kinase inhibitors (TKIs).

METHODS We analyzed 1,431 NSCLC patients who were treated with either gefitinib or erlotinib. Exons 18 to 21 of EGFR were analyzed by PCR and subjected to direct sequencing methods.

RESULTS Of 306 patients who had EGFR mutation, 24 patients (7.3 %) had complex mutations. The frequency of rare mutations was 10.3 %. Four groups were categorized [group A (N = 269): classical mutation alone; group B (N = 16): complex mutation with classical mutation; group C (N = 16): rare mutation alone or complex mutation with rare mutation; group D (N = 5); classical mutation with T790M]; the response rate (RR) to TKI was significantly different between each group (RR = 74.8 % in group A vs. 68.8 % in group B vs. 25.0 % in group C vs. 80.0 % in group D, P < 0.001). Progression-free survival (PFS) was also poorer in rare mutations (median PFS: 11.9 vs. 8.1 vs. 1.4 vs. 8.0 months, respectively, P < 0.001).

CONCLUSIONS NSCLC patients harboring rare mutations did not show consistent and favorable responses to EGFR TKI compared with those harboring classical mutations. However, complex mutations with classical mutations showed similar treatment efficacy toward EGFR TKI to that with classical mutations alone.