At present, the evidence for the role of cfDNA in monitoring treatment outcomes comes mainly from lung cancer, with the EGFR exon 20 p.T790M mutation leading to EGFR tyrosine kinase inhibitor resistance being reliably detected in the plasma cfDNA, 16 to 49 weeks before clinical or radiological progression is detected [74,75,76]. Here, EGFR is linked to lung cancer.