@abob, to add to what Lisa said, "ctDNA not detected" does not mean you don't have any circulating cells. This means there aren't enough circulating cells to exceed the minimum detection level. I've received many reports of no circulating cells, but I keep getting tested because I've had lung cancer that has metastasized to my brain. ctDNA is just one piece of information your doctors combine with everything they know about their patients to get the whole picture. As I say in my presentations, after your first lung cancer diagnosis, it's "Welcome to the world of testing!"

This is also why Dr. Google and Dr. YouTube are often wrong or misleading. They know nothing about the patient's history, lifestyle, personal habits, etc., which are essential to informing a doctor's picture of what's happening.