EGFR exon 20 insertion testing identifies a distinct subset of lung adenocarcinomas, accounting for at least 9% of all EGFR-mutated cases and by molecular modeling, are predicted to have potentially different effects on erlotinib binding. Studies show that in contrast to the more classic activating EGFR mutations, these insertions have been associated with de novo resistance to approved EGFR-TKIs (erlotinib and gefitinib). In a recent study, patients with advanced lung adenocarcinoma harboring exon 20 insertions demonstrated no response or partial response following treatment with TK inhibitors. Exon 20 insertion mutations in EGFR may be associated with clinical trials (https://clinicaltrials.gov/).

In GBM, EGFR mutations typically cluster in the extracellular domain and include in-frame deletions (such as the common “variant III” del 6-273) and missense mutations (A289V, A289D, T263P, G598V). EGFR exon 20 insertions have not been previously reported in GBM. The clinical significance of this mutation with regards to response to anti-EGFR therapy in GBM is unknown. In general, EGFR exon 20 mutations have been reported in approximately 9% of all EGFR-mutated cases of lung cancer and studies show that in contrast to the more classic activating EGFR mutations, these insertions have been associated with de novo resistance or only partial response to approved EGFR-TKIs (erlotinib and gefitinib) in lung cancer.