Circulating Tumor DNA Response and Minimal Residual Disease Assessment in DNA Polymerase Epsilon-Mutated Colorectal Cancer Undergoing Immunotherapy.

Exonuclease domain mutation (EDM) in polymerase epsilon ()-mutated colorectal cancer patients is characterized by specific clinical features and a very high tumor mutation burden (TMB). The therapeutic effectiveness of immune checkpoint inhibitors (ICIs) for the treatment of colorectal cancer in patients with mutations is poorly defined. Our case represents a young-onset colon cancer patient who has had a continued response to programmed cell death protein 1 (PD1) blockade alongside clearance of circulating tumor DNA (ctDNA) using a tumor-informed approach. Utilizing ctDNA kinetics to assess minimal residual disease (MRD) in the context of colorectal cancer is a very important topic. Furthermore, utilizing ctDNA kinetics in response to immunotherapy is something that is relevant to all tumor types undergoing immunotherapy. Recently, several landmark articles have proposed this as a promising approach. There is, however, limited information in the literature showing the feasibility of such an approach. Our case report is going to be of value, both from a scientific as well as a clinical standpoint. This is particularly relevant given the rise of colorectal cancers in young individuals.