Study of circulating tumor DNA (ctDNA) of RAS-wild metastatic colorectal cancer (mCRC) as a tool for on-site mutation screening: A prospective study

e15077 Background: Clinical use of ctDNA has been become an important criterion for patient care. The transfer of this knowledge and technology from top clinical labs to local hospitals is still distant. With the goal of a robust, precise, simple and cost-effective genomic test, we designed a prospective study featuring high-sensitive mutation screening of ctDNA from RAS-wild mCRC with digital PCR (dPCR) technology. This approach allows detection of 85 hotspot mutations of KRAS, NRAS, BRAF and PIK3CA, which are present in approximately 60% of CRC, and is clinically important information for anti-EGFR treatment. Methods: 18 “ RAS-wildtype” mCRC patients, initially diagnosed with PCR-rSSO (RASKET) using CRC FFPE samples, were recruited. After extracting ctDNA from patient plasma, dPCR analysis was conducted with LBx Probes. Results: Mutations in KRAS, NRAS, BRAF and PIK3CA, were detected in 5 samples ( KRAS in 3 samples, NRAS in 1 sample, BRAF in 1 sample, and PIK3CA in 2 samples). In a patient undergoing treatment with panitumumab, two different KRAS clones were detected, indicating possible clonal evolution under drug pressure. Interestingly, we also encountered another patient who had never received anti-EGFR antibodies, but who tested RAS mutation-positive at a level below the detection sensitivity of PCR-rSSO. Conclusions: Our dPCR screening with ctDNA was able to identify mutations successfully in a previously-diagnosed “ RAS-wild” mCRC group. Based on these results, we designed the following test, which with