Your risk assumptions seem to revolve around invasive testing to confirm scan results. IE: needle biopsy or even surgical biopsy.
Using my brother as the example, there were NO invasive tests performed to verify the scan. There were several other scans, most of them with a radioactive component. CT's and PET's for example. The PET scan, in particular, totally confirmed the kidney cancer diagnosis; the tumor lit up like a spotlight due to the radioactive component injected.
So there are ways to check it out without any invasive testing.
Further, if you get a "baseline" scan an it showed a small pea sized growth on a kidney, you could always just "standby" and repeat the scan in 6 months or so. If there was no change, leave it alone. If it was the size of an egg.... well, it NEEDS checking.
I think as time goes by, these scans are going to be recognized for the valuable tool that they are. Because of the heart disease in my family, my sons are going to get baseline heart scans starting at age 25 and repeat every 3-5 years or as necessary. To me, it's really cheap insurance.
I feel the same about a body scan. Get the baseline and see if there's anything that needs watching. Repeat scan at a suitable time if there is something to watch.
As I said..... $500? You can't get cr*p done in the way of overall testing in a regular Doctor's office for $500.
But each to their own. For my family, it's been just about a miracle. The ability to non-invasively "see" a tumor before it metastasized into a fatal problem...... wow, we live in wonderful times.