Circulating Tumor DNA (ctDNA) Clearance After Immune Checkpoint Inhibition (ICI) with Radiotherapy (RT) as a Prognostic Biomarker for Advanced Melanoma

Background/Objectives: ctDNA is evolving as an important biomarker for prognostic assessment in advanced melanoma. However, the utility of ctDNA monitoring during concurrent ICI with RT remains incompletely explored. Methods: In this multicenter retrospective study, patients with unresectable stage III/IV melanoma treated with ICI and palliative RT were identified. Patients had retrospectively collected, tumor-informed, exome-based, ctDNA monitoring within 3 months following RT. Results: A total of 50 patients treated with ICI and palliative RT were analyzed. Compared with ctDNA clearance, ctDNA detectable and increasing was associated with worse OS (adjusted HR 2.52, 95% CI 1.20–5.29; p = 0.015), whereas ctDNA detectable and decreasing was not significantly different from ctDNA clearance (adjusted HR 1.58, 95% CI 0.69–3.65; p = 0.282). One-year OS was 87.5% with persistently undetectable ctDNA, 83.3% for ctDNA clearance, 46.3% for detectable and decreasing ctDNA, and 21.4% for detectable and increasing ctDNA. In a multivariable analysis, detectable and increasing ctDNA was associated with worse OS compared with ctDNA clearance (HR 2.52, 95% CI, 1.20–5.29, p = 0.015). Conclusions: Among patients with advanced melanoma treated with ICI and palliative RT, ctDNA clearance is associated with significantly improved OS compared to patients with detectable and increasing ctDNA within 3 months following RT. Ongoing prospective studies are needed to understand the role of ctDNA in this population.