Safety and Feasibility of PD-1 Blockade in the Treatment of dMMR or MSI-H Rectal Cancer

This study investigates the safety, tolerability, and feasibility of Pembrolizumab, an immunotherapy agent, in combination with SOC radiation and Capecitabine (or 5-Fluorouacil) in treatment of patients with mismatch repair deficient locally advanced rectal cancer. Pembrolizumab is an investigational (experimental) drug that works by enhancing the functional activity of the target lymphocytes (immune cells) to facilitate tumor regression and ultimately immune rejection. Pembrolizumab in combination with radiation and Capcitabine (or 5-Fluorouacil) is experimental because it is not approved by the Food and Drug Administration (FDA) for this specific indication. The primary objective of this study is to determine the safety, tolerability and feasibility of neoadjuvant pembrolizumab in combination with capectiabine (or 5-Fluorouracil ) in the treatment of patients with MMR-d locally advanced rectal cancer The secondary objective of this study is to determine the treatment response in MMR-d rectal cancer patients treated with neoadjuvant chemoradiotherapy and Pembrolizumab.