Abstract
The standard of care for stage III colorectal cancer (CRC) is curative resection with adjuvant chemotherapy (ACT). There is a high risk of recurrence particularly for high-risk patients with stage III disease, making close disease monitoring vital. Circulating tumor DNA (ctDNA) is now established as an effective method of early detection of disease relapse as well as postoperative risk stratification. However there remains a lack of established protocol for using ctDNA to assess response to ACT and in using that data to alter therapy in real time. A case is described of a patient with high-risk stage III CRC in whom failure of ACT was detected early and therapy was quickly changed based on rising ctDNA levels. The described patient had complete radiologic and clinical response to checkpoint inhibitor immunotherapy and remains free of disease after 18 months. This case demonstrates a promising example of how ctDNA can be used to both assess effectiveness of ongoing therapy and drive real-time change in treatment while sparing unnecessary chemotherapy toxicities.