Analysis of ctDNA assay as a strategy for watchful waiting in locally advanced esophageal cancer.

e16126 Background: Standard treatment for locally advanced esophagogastric (EG) cancer is neoadjuvant chemoradiotherapy (CRT) followed by surgery. The treatment goal is curative in order to control the tumor in the thorax, prevent regional recurrence, and to help prevent or delay metastatic progression. Despite achieving pathologic complete response (pCR) after surgery, patients with supposed local control may have recurrence up to a third of the time. Circulating tumor DNA (ctDNA) is an evolving tool in the field of oncology used as an informative biomarker. We provide longitudinal, real world observation of ctDNA minimal residual disease (MRD) to predict pCR and risk of recurrence. Methods: 22 participants with newly diagnosed locoregional EG cancer were enrolled under single institution approved protocol IRB#1864127-7. CtDNA-based MRD levels were obtained by Natera SignateraTM bespoke mPCR-NGS assay. Additionally, Natera Altera TM was used as a non-invasive biopsy to provide the comprehensive tumor-specific genomic profile. We evaluated thirteen participants who underwent definitive surgery and those who did not. Non-invasive plasma samples were surveilled at various timepoints which included the time of diagnosis, during neoadjuvant CRT, after definitive surgery. The primary outcome is a comparison of assay recurrence (AR) to disease recurrence (DR) defined as radiologic or clinical disease recurrence in participants with locoregional disease. Exploratory endpoints examine the profile of tumor. Results: Baseline levels were detected (n = 22) at new diagnoses. Longitudinal results demonstrated ctDNA clearance in eight participants while undergoing neoadjuvant chemoradiaton. Residual ctDNA were found in five participants for which two had clinical disease recurrence. Esophagectomy was done in three participants achieving pCR. Post procedural ctDNA were detected in one participant. Conclusions: The clearance of ctDNA favors less clinical disease recurrence in this study. This may indicate that ctDNA clearance confers lower risk of relapase. However, a larger cohort prospective study will be needed to validate the application of ctDNA as an early biomarker. The future application of ctDNA can be used to identify patients who may benefit from a watchful waiting adjuvant strategy instead of proceeding with invasive esophagectomy.