Detection of <i>EGFR</i> T790M Mutation by Droplet Digital Polymerase Chain Reaction in Lung Carcinoma Cytology Samples.

Abstract

Advanced-stage non-small cell lung carcinoma patients on EGFR-targeted tyrosine kinase inhibitors frequently present with an acquired <i>EGFR</i> T790M resistance mutation. Early detection using a high-sensitivity assay is critical to allow patients to switch to third-generation tyrosine kinase inhibitors. The detection of <i>EGFR</i> T790M mutation is often challenging because of low tumor fraction in posttreatment specimens. Because a large fraction of non-small cell lung carcinoma patients are given a diagnosis by cytology, evaluating a high-sensitivity technique for <i>EGFR</i> T790M detection in these specimens is essential. To evaluate a high-sensitivity droplet digital polymerase chain reaction (ddPCR) assay for <i>EGFR</i> T790M using different cytologic specimen preparations. A total of 42 cytology samples, including smears and cell block preparation, were evaluated for <i>EGFR</i> T790M using ddPCR. The results of the mutation assay were compared to the patient's known <i>EGFR</i> T790M mutation status. The ddPCR assay successfully determined the <i>EGFR</i> T790M mutation status in 36 of 42 samples (86%), including samples with low tumor fraction (≤20%). In 4 cases the results of the ddPCR assay could not be compared because the mutation status was unknown at the time of collection of the cytology sample. There was 1 false-positive result, with borderline positivity, and 1 false-negative result. Overall sensitivity and specificity of the ddPCR assay were 93% and 96%, respectively. Our results indicate that <i>EGFR</i> T790M ddPCR is a highly sensitive and specific mutational assay that can be used reliably in cytologic specimens, including samples with low tumor fraction.