In addition to being a powerful biomarker to enable personalized cancer therapy, liquid biopsies provide a unique window into tumor biology that can be sampled at multiple time points over the course of the natural history of disease and throughout therapy. Recent studies have highlighted the substantial genetic heterogeneity that can exist with human tumors
. The accumulation of mutations, genetic drift, and selective pressures within the tumor microenvironment can lead to tumor evolution that can impact how cancers respond to treatment
. Multi-region sequencing has suggested that early tumor development may generally occur under weak evolutionary pressures
but that selection appears to increase at later stages of tumor development and metastasis
. However, the impact of intratumoral heterogeneity on radiotherapy outcomes remains to be defined. Modeling experiments suggest that subclones with heterogenous radiosensitivity could be a major determinant in treatment response
. Further, although there is strong evidence that systemic therapy selects resistant subclones that lead to disease progression
, the effect of radiotherapy on tumor evolution in acquired resistance to radiotherapy is less clear. The sequencing of pre- and post-treatment samples from patients undergoing chemoradiotherapy for rectal cancer suggested that radiotherapy can substantially impact the clonal architecture and can potentially select resistant subclones
. Additionally, retrospective clinical studies have reported worse local control after reirradiation compared to initial treatment with radiotherapy
, but these studies are confounded by lower doses of radiation in the reirradiation setting due to concern for toxicity.
Two major limitations to studying clonal evolution during radiotherapy are (1) sampling biases related to analyzing one or a few regions within a patient’s tumor and (2) a lack of longitudinal specimens to study clonal dynamics. ctDNA has the potential to be released from all of the tumor cells in a patient and can be sampled longitudinally from routine blood draws, making it a powerful tool to characterize clonal evolution (Figure 1
). Multiple studies have applied ctDNA analysis to monitor clonal evolution during the treatment of patients with systemic therapy. For example, analysis of paired pre-treatment and end-of-treatment ctDNA samples demonstrated clear evidence of resistant subclone selection during the treatment of metastatic estrogen receptor-positive breast cancer with CDK4/6 inhibition and endocrine therapy
. In addition, ctDNA analysis demonstrated that subclonal tumor populations contribute to relapse after surgery and adjuvant chemotherapy for early stage lung cancer
. As a result, longitudinal ctDNA analysis and subclone tracking could help to clarify the modes of tumor evolution during radiotherapy.