Clinical applications of extended ctDNA RAS mutation determination in metastatic colorectal cancer.

607Background: Tumor tissue is currently used for RAS testing in metastatic colorectal cancer (mCRC) patients, but detection of circulating tumor (ct) DNA in plasma is being actively investigated as an alternative for detection and monitoring RAS mutations during therapy. Methods: Concordance of RAS testing in plasma using the OncoBEAM RAS CRC test and RAS testing in tissue using standard-of-care RAS detection techniques was evaluated in 114 mCRC antiEGFR-naive patients. In discordant cases, tissue samples were re-examined by BEAMing. OncoBEAM RAS CRC assay was also used to monitor RAS mutation status in serial plasma samples from 34 patients treated with anti-VEGF +/- chemotherapy or anti-EGFR based therapy. Results: The overall agreement of the two methods was 93% (106/114 patients), with positive agreement of 96.2% (51/53) and negative agreement of 90.2% (55/61). Longitudinal monitoring of plasma samples from 13 patients with RAS mutated tumors treated with chemotherapy +/- anti-VEGF, showed a signific...