Background: Tumour heterogeneity is a key determinants of cancer resistance. Serial sampling of cell free (cf)DNA may detect evolving somatic mutations. Monitoring cancers after therapies e.g. selective internal radiation therapy (SIRT) require new biomarkers as RECIST imaging was developed to assess response to chemotherapy and may not reflect tumour changes due to new therapeutic strategies. The utility of cfDNA to detect recurrence and predict response is emerging. The objective of this study was to sequence serial cfDNA samples from patients with liver predominant metastatic disease receiving SIRT, and explore the feasibility of using this method to detect evolution of somatic mutations after high dose radiation. Methods: A prospective imaging biomarker study was performed in patients with colorectal liver metastases (CRLM) receiving SIRT. Plasma was extracted and frozen within 4 hours at 3 time points: baseline, 4 and 10 weeks after SIRT. Ion Torrent Amplicon sequencing was performed using cancer hotspot panel v.2. Sequencing of primary tumours was obtained by pyrosequencing. Results: Twenty-four patients with CRLM were recruited from March – Dec 2015, and 18 had cfDNA extracted for sequencing at a minimum of two time points. Ion Torrent amplicon sequencing of baseline cfDNA showed high concordance with formalin-fixed paraffin-embedded (FFPE) tumour samples. Serial cfDNA sequencing in a patient with partial response by imaging, detected 22% KRAS G12C mutation at baseline, which decreased to 1.5% by 4 weeks, then was undetectable 10 weeks after SIRT. Sequencing of another patient’s cfDNA revealed persistence of KRAS G13D and a truncating APCmutation at both 4 and 10 weeks after SIRT, consistent with the patient’s progressive disease. Conclusions: Cell-free DNA is emerging as a biomarker in colorectal cancer. Our measurements of mutational status in baseline cfDNA and FFPE samples show high concordance. This study reports that serial cfDNA sequencing detects changes in mutational status and specific mutations following high dose radiation to the liver. This adds to the evidence of cfDNA as a tool to detect evolution of somatic mutations following therapy.