McEvoy AC, et al. - In this study involving treatment-naïve metastatic melanoma patients, researchers studied the relationship between circulating tumor DNA (ctDNA) levels and the patients’ overall metabolic tumor burden (MTB). They used 18F-fluoro-D-glucose positron emission tomography/computed tomography (FDG PET/CT) to measure MTB and metabolic tumor volume (MTV). They also used droplet digital PCR (ddPCR) to quantify plasma ctDNA. Findings revealed a relationship between ctDNA levels measured by ddPCR and MTB. A limit in tumor size for which ctDNA cannot be detected in blood was also observed. Nevertheless, they concluded that ctDNA can be used as a non-invasive corresponding modality to functional imaging for monitoring tumor burden.