That does sound really frustrating. I'm not a doctor, but have some thoughts. Cost/insurance could be an issue for all, and it is also possible that they aren't realistic for your cancer type (which seems to be slow-growing).

One is trying to ask for a PET scan. I don't know if the type of cancer you have had is likely to show up on a PET scan, but sometimes a PET scan can distinguish between a cancer and scar tissue. I've been told that insurance tends not to approve PET scans, at least for endometrial cancers (what I have).

Another is the Signatera test from the company Natera. They would need a sample of your 2017 tumor so that they can sequence its DNA. They then develop a personalized blood test that looks for DNA mutations found in your tumor. Sometimes DNA released from the tumor circulates in your blood. Being able to detect the cancer mutation suggests the cancer is still there. But a slow-growing tumor may not release much DNA, so this test might not detect it. I doubt insurance would pay.

If you're really worried, another conceivable possibility would be a CT-guided needle biopsy of the "scar tissue." It would have to be in a place that's accessible. And insurance would probably balk.

There have been lots of cancer breakthroughs in the last 17 years, but unfortunately they have not been in detecting very low levels of cancer cells. People are trying, but it's actually really hard. Tests either have significant levels of false negatives (like Signatera) or they have false positives, which would lead to unnecessary worry and treatment.