Cell-free DNA (cFDNA) in serum was isolated and digested with methylation-sensitive restriction enzymes (Bsh1236I, HpaII and HinP1I) to quantify the amount of methylated Adenomatosis-poliposis-coli gene (APC), Gluthation-a-transferase-protein 1 gene (GSTP1), ARF tumor suppressor protein gene (p14(ARF)), cyclin-dependent kinase inhibitor 2A (p16), Retinoid-acid-receptor-beta gene (RAR-B), RAS-association domain family-1 gene (RASSF1), Tissue inhibitor of metalloproteinase-gene (TIMP3) and Prostaglandin-endoperoxid synthase 2 (PTGS2) DNA fragments.