Background: Circulating tumor DNA (ctDNA) has emerged as a biomarker for non-invasive longitudinal monitoring of tumor progression in cancer management. Through the recent advances in next generation sequencing (NGS) technologies and personalized assays, ctDNA has been heralded as a promising tool to detect residual disease, relapse, and monitor treatment response in hematologic malignancies and solid tumors. Our study aims to determine whether treatment related ctDNA dynamics can be used as a reliable indicator to predict pathologic complete response (pCR) in patients with rectal cancer receiving neoadjuvant treatment. Methods: This is a prospective observational cohort study. The primary aim is to estimate the sensitivity and specificity of ctDNA clearance in predicting pCR in patients undergoing neoadjuvant therapy. The secondary aim is to evaluate the feasibility of using ctDNA as a surveillance method to detect progression of rectal cancer during neoadjuvant therapy and relapse in the subsequent follow up period. ctDNA levels are collected from newly diagnosed rectal cancer patients at 7 discrete time points: at diagnosis or screening, during neoadjuvant therapy, after completion of neoadjuvant therapy and 1 month, 2 months, 4 months, 6 months after surgery. This will be followed by every 3 months ctDNA testing for surveillance for up to 2 years. We expect to enroll approximately 30 patients at our institution. The subjects will be sorted into two groups: responders and non-responders based on whether they achieve pCR. ctDNA level between two groups will subsequently be compared. The use of ctDNA to predict pCR in rectal cancer patients may allow for many of these patients to safely avoid surgery if undetectable ctDNA at the end of neoadjuvant therapy strongly correlates with pCR. We are actively enrolling patients into this prospective observational study and expect to report the data in the near future. The data we obtain will be combined with those from several institutions around the US doing similar studies with the same test. Data analysis will subsequently be conducted.