Dynamics and utility of cell-free DNA release following radiotherapy in prostate cancer and non-small cell lung cancer

INTRODUCTION: The tissue-of-origin of plasma cell-free DNA (cfDNA) can be determined by methylation analysis and circulating tumour DNA identified by next-generation sequencing (NGS). To evaluate the dynamics of cfDNA during radical radiotherapy, total cfDNA was quantified in patients undergoing radiotherapy for prostate and lung cancer; samples from a sub-cohort underwent methylation analysis and NGS. METHODS: Analysis of blood plasma cfDNA was carried out on samples from patients receiving stereotactic ablative body radiotherapy for non-small cell lung cancer (n=39), conventional radiotherapy for prostate cancer (n=59), and stereotactic radiotherapy for localised prostate cancer (n=19). Radiotherapy doses delivered to healthy tissues, survival outcomes and treatment toxicity outcomes were evaluated. Samples from three lung patients underwent NGS, and samples from 11 prostate patients underwent methylation analysis. In the lung cancer patients, radiation-induced lung injury (RILI) was measured on follow-up computed tomography scans and correlation analysis carried out between the dose received by normal lung and the relative volume of RILI. Prostate cancer patients completed a questionnaire evaluating their attitudes to the risk of treatment toxicity. RESULTS: Total cfDNA concentration was not predictive of treatment outcomes. Somatic lung cancer-associated mutations were detected by NGS of cfDNA in 2/3 patients. The dose delivered to lung tissue correlated with the volume of RILI on post-treatment CTs. In the prostate cancer cohort, methylation analysis showed an increase in prostate tumour-derived cfDNA signal during radiotherapy in all 11 prostate cancer patients included in the sub-cohort. There was an increase in bladder and rectum-derived cfDNA signal in 2/5 and 1/5 patients respectively. The treatment questionnaire in prostate patients indicated their unwillingness to accept more severe side effects, regardless of the theoretical chance of cure. DISCUSSION: CfDNA released during prostate radiotherapy may provide a means of detecting tumour and normal tissue response to radiotherapy. Sequencing analysis of cfDNA can identify tumour-related mutations in patients with early-stage lung cancer. Dosimetric data and analysis of cfDNA may help predict the development of RILI, though studies in larger numbers are needed. The risk of post-treatment toxicity is an important consideration for patients