622P - Liquid biopsy concordance based on clonality and timing of testing in patients with metastatic colorectal cancer

Abstract

BackgroundThe most recent consensus statement on the use of liquid biopsies (circulating tumor DNA – ctDNA testing) has been that it is not yet ready for prime time. However, in patients with metastatic colorectal cancer (mCRC), there is significantly more ‘shedding’ of DNA detectable in blood allowing this test to be of value. We aimed at reporting the concordance of liquid biopsies based on clonality and timing of testing in patients with mCRC. MethodsA total of 92 mCRC patients were identified who had both a commercially available liquid and tissue next generation sequencing assay done from December 2016 to February 2019. Arbitrarily, mutations were classified as clonal or subclonal based on the 50% cutoff of the highest variant allele frequency (VAF) reported. Concordance rates, including clonal (BRAF-V600E/ RAS), subclonal and amplification concordance were calculated separately for patients for whom the liquid biopsy testing was done before initiation of treatment (n=27) and after initiation of treatment (n=65). ResultsClonal concordance rates were 96.3% for patients when the liquid biopsy was done before initiation of treatment versus 64.6% for patients when the test was obtained after they were already on some systemic therapy (p value: 0.001). Similarly, subclonal and amplification concordance rates for patients in the test before treatment and test after treatment groups are summarized in the table. Moreover, the median of highest VAF% was noted to be 3.1% and 1.1% in test before treatment and test after treatment groups respectively (p value: 0.092).Table622P Summary of concordance rates by timing of liquid biopsy testTableTest before treatment (N=27)Test after treatment (N=65)Total (N=92)P value*Clonal Concordance26: 96.3 (81.0, 99.9)42: 64.6 (51.8, 76.1)68: 73.9 (63.7, 82.5)0.001BRAF27: 100.0 (87.2, 100.0)64: 98.5 (91.7, 100.0)91: 98.9 (94.1, 100.0)1.000RAS26: 96.3 (81.0, 99.9)52: 80.0 (68.2, 88.9)78: 84.8 (75.8, 91.4)0.058Subclonal Concordance21: 77.8 (57.7, 91.4)44: 67.7 (54.9, 78.8)65: 70.7 (60.2, 79.7)0.452BRAF27: 100.0 (87.2, 100.0)65: 100.0 (94.5, 100.0)92: 100.0 (96.1, 100.0)1.000RAS27: 100.0 (87.2, 100.0)62: 95.4 (87.1, 99.0)89: 96.7 (90.8, 99.3)0.553Amplification Concordance22: 81.5 (61.9, 93.7)41: 63.1 (50.2, 74.7)63: 68.5 (58.0, 77.8)0.092Highest Variant Allele Frequency (VAF)%3.1 (0.0, 84.6)1.1 (0.0, 90.1)1.5 (0.0, 90.1)0.092Concordance rates are summarized by count: percent (95% binomial confidence interval). P values arise from Fisher’s exact tests; *Arbitrarily, 50% of the highest variant allele frequency (VAF)% value in the liquid biopsy results has been used to differentiate clonal from subclonal mutations. Highest VAF% is summarized by median (range). P values arise from Wilcoxon rank sum test ConclusionsLiquid biopsies show a very high concordance rate in patients with metastatic colorectal cancer. It is important to take the timing of the assay into consideration alongside relevant clonal mutations while assessing the concordance of liquid biopsies, not just for mCRC but for other malignancies. Legal entity responsible for the studyThe authors. FundingHas not received any funding. DisclosureAll authors have declared no conflicts of interest.