NOAC9 - Circulating Tumor DNA Guided Follow-Up in Anal Cancer

This study is investigating whether a blood test that detects cancer DNA (called circulating tumor DNA or ctDNA) can help guide follow-up care for people with anal cancer after treatment. The goal is to see if this simple blood test can detect cancer returning earlier than standard imaging. **You may be eligible if...** - You have been diagnosed with squamous cell carcinoma of the anus (anal cancer) - You are eligible for definitive treatment with radiation therapy (with or without chemotherapy) - You are 18 or older - You can provide written and verbal consent **You may NOT be eligible if...** - You have a condition preventing blood draws - You have a condition preventing a PET-CT scan - You are unlikely to be able to comply with the follow-up program Talk to your doctor to see if this trial is right for you.