Personalized Circulating Tumor DNA Analysis for Predicting Outcomes and Tracking Response to Radiotherapy and Pembrolizumab in Localized Sarcomas: Analysis of the SU2C-SARC032 Trial

PURPOSE Circulating tumor DNA (ctDNA) holds promise for prognostication and disease monitoring in solid tumors. However, rare and heterogenous tumors with low mutational burden like soft tissue sarcomas (STS) present a technical challenge for ctDNA analysis. We investigated the ability of personalized ctDNA analysis to predict outcomes and track response to radiotherapy and pembrolizumab in patients with localized STS. METHODS We analyzed 313 plasma samples collected pretreatment, postradiotherapy/presurgery, and postsurgery from 106 patients treated with preoperative radiotherapy with or without perioperative pembrolizumab for high-risk, localized STS in the SU2C-SARC032 phase II randomized trial. Personalized ctDNA assays were designed to track a median of 46 somatic mutations per patient. Associations between ctDNA levels and outcomes were assessed using univariable and multivariable models adjusting for clinical risk factors. RESULTS Personalized ctDNA profiling detected baseline ctDNA in 85% of patients. Baseline ctDNA levels correlated with tumor size, grade 3 disease, and expression of genes related to hypoxia and the cell cycle. ctDNA levels pretreatment, after radiotherapy with or without immunotherapy but before surgery, and postsurgery were strongly associated with disease-free and overall survival on univariable and multivariable analyses. Patients treated with pembrolizumab, who had significantly better disease-free survival, had a greater decrease in their ctDNA levels after neoadjuvant therapy. Finally, personalized ctDNA analysis enabled noninvasive subclone tracking and patients with multiple tumor clones detected by ctDNA analysis after surgery had inferior outcomes. CONCLUSION Tumor-informed, personalized profiling enables sensitive ctDNA detection and quantitative monitoring in localized STS, providing independent prognostic information across the perioperative course. These findings support prospective ctDNA-guided interventional trials in STS.