Liquid Biopsy to Identify Actionable Genomic Alterations.

Liquid biopsy has been used extensively in solid malignancies to detect actionable driver mutations, to monitor treatment response, to detect recurrence, to identify resistance mechanisms, and to prognosticate outcome. Although many liquid biopsy sequencing platforms are being used, only five test kits have received government approval. We review representative literature on these government-approved liquid biopsy kits, which are primarily used to detect EGFR mutation in lung cancer and RAS ( KRAS, NRAS, BRAF) mutations in colorectal carcinoma. Another emerging use of single-gene liquid biopsy is to detect PIK3CA mutations and to understand resistance to hormonal blockade in breast and prostate cancers. The two most commonly used next-generation sequencing (NGS) liquid biopsy tests (Guardant 360, Guardant Health; FoundationACT, Foundation Medicine Inc.) are discussed. The ability and the applicability of NGS platform to detect tumor mutation burden are also addressed. Finally, the use of circulating tumor DNA (ctDNA) to detect minimal residual disease may be the most important use of ctDNA in the setting of tumor heterogeneity. The ability to identify "shedders" and "nonshedders" of ctDNA may provide important insight into the clinicopathologic characteristics of the tumor and portend important prognostic significance regarding survival.