This article has been updated to add comments from Amgen about the exploratory analysis presented at ASCO. CHICAGO – Using circulating tumor DNA (ctDNA) testing proved to be an effective tool to monitor response and potentially guide treatment decisions in an exploratory analysis of a Phase III trial assessing Amgen's Lumakras (sotorasib) plus Vectibix (panitumumab) in KRAS G12C-mutant metastatic colorectal cancer.