III期结直肠癌（CRC）的标准治疗是行根治性切除手术并辅以辅助化疗（ACT）。尤其对于高危患者，III期疾病有很高的复发风险，因此密切监测疾病非常重要。现已确立外周血循环肿瘤DNA（ctDNA）作为早期检测疾病复发以及术后风险分层的有效方法。然而，目前还缺乏使用ctDNA评估ACT疗效以及根据这些数据实时调整治疗的成熟方案。本文描述了一例高危III期结直肠癌患者，早期检测到ACT失败，并根据上升的ctDNA水平迅速改变了治疗方案。该患者经过检查点抑制剂免疫疗法，出现了完全的放射学和临床反应，在18个月后仍无疾病复发。该例表明了ctDNA如何在评估正在进行的治疗的有效性以及推动实时改变治疗而减少不必要的化疗毒性方面具有良好的前景。© 作者（或其雇主）2023年。在CC BY-NC许可下可重复使用。不可商业复制。由BMJ出版。

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.© Author(s) (or their employer(s)) 2023. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.