Circulating Tumor DNA (ctDNA) as a Predictive Biomarker for Immunotherapy in Advanced or Locally Advanced dMMR/MSI-H Colorectal Patients

This study is testing whether a blood test that detects tumor DNA fragments (called circulating tumor DNA, or ctDNA) can predict how well immunotherapy works in people with advanced colorectal cancer that has a specific genetic feature called dMMR or MSI-H. This feature makes the cancer more likely to respond to immune checkpoint drugs. **You may be eligible if...** - You have advanced or locally advanced colorectal cancer with the dMMR or MSI-H genetic marker - You are receiving or are expected to receive immunotherapy (immune checkpoint inhibitor) - You are in reasonable health (ECOG performance score 0–1) with a life expectancy of at least 12 weeks **You may NOT be eligible if...** - You have another untreated cancer - You have serious medical conditions that interfere with study participation - You have an autoimmune condition that makes you unsuitable for PD-1 immunotherapy Talk to your doctor to see if this trial is right for you.