Molecular residual disease (MRD) detection using bespoke circulating tumor DNA (ctDNA) assays in localized soft tissue sarcoma (STS): A multicenter study.

11537 Background: Surgery and (neo)adjuvant radiotherapy (RT) are the mainstay curative treatments for localized STS. Despite treatment, up to 50% of STS patients experience metastatic relapse, and routine use of adjuvant systemic therapy (AST) remains controversial. The presence of ctDNA following curative-intent treatment of STS is a potential biomarker for MRD and may identify patients who are likely to benefit from AST. Given the genomic heterogeneity of STS, a histology-agnostic approach to ctDNA detection in this population is desirable. Methods: This multicentre prospective study enrolled patients with localized, high-risk (grade ≥ 2, size ≥ 5 cm) STS. Blood samples were collected at diagnosis, post- radiotherapy, post-surgery, and at serial longitudinal time points for up to 2 years. Standard radiologic follow-up was performed concurrently. Whole exome sequencing of tumor tissue was carried out to identify patient-specific, single nucleotide variants. Personalized, and tumor-informed multiplex PCR next-generation sequencing-based ctDNA (Signatera) assays were used to track ctDNA in serial plasma samples. The primary endpoint was ctDNA detection rate of >70% at diagnosis. Secondary endpoints were MRD detection after local therapy and correlation of ctDNA detection with disease relapse. Results: A total of 76 subjects (female n = 43; median age range: 58 21- 84 years) were included in this study from Princess Margaret Cancer Center, Stanford University, and Emory University. The most common STS types observed were leiomyosarcoma (LMS, n = 28), liposarcoma (n = 14), (predominantly dedifferentiated n = 6), and myxofibrosarcoma (n = 11). Among 38 pts who had blood samples collected at baseline (time of surgery), ctDNA was positive in 30/38 (79%). A baseline sample prior to radiotherapy was collected in 24 pts. Following neoadjuvant radiotherapy, 8/24 pts (33%) who were ctDNA positive at baseline became ctDNA negative, while 29/30 became ctDNA-negative after surgical resection. Median follow-up was 19 months, and 19/76 pts (25%) experienced disease recurrence Among these 19 pts, ctDNA was detected in all pts with baseline studies. ctDNA was detected at or before radiologic recurrence in 9/19 pts (47%), predominantly LMS 4/9, with a median lead time of 64.8 days (range: 0-197 days). ctDNA was detected in 14/38 patients at baseline with no radiological recurrence during follow-up period. Conclusions: Personalized, tumor-informed ctDNA assays can detect MRD and has prognostic value after definitive therapy, surgery and (neo)adjuvant radiotherapy, in localized STS patients. Additional studies are ongoing to evaluate ctDNA as a predictive biomarker for benefit from AST in STS. Clinical trial information: NCT03818412 .