Insurance denial PET?, New Mets Diagnosis

Helllo.  I have a question or two about how to handle it when an insurance company denies coverage for something that seems clearly to be medically necessary from my doctor's viewpoint.

I have been post-breast cancer (triple negative) (right side) treatment for about 1.5 years and about 1 month ago, my pulmonologist, who was doing follow up CT scans of the lungs as part of treatment for pneumonitis secondary to radiation treatment, found a nodule in my right lung that was not there 4  months earlier. 2 weeks later I had a needle biopsy, which collapsed my lung   The biopsy was positive for cancer, but not clear which part of the body the cancer was from, though it had hormone negative characteristics, making it seem likely it is metastasis of the BC.

Before the biopsy, my pulmonologist had ordered a PET scan  which was scheduled after the biopsy, but my insurance denied coverage. With the new cancer diagnosis, my oncologist then ordered a PET scan and I scheduled it for a week after the cancelled one. The insurance denied it again, but this time I didn't let the hospital cancel it and told them I would self-pay. I knew that we needed this scan to see if there were other tumors and if so, where and it wasn't time to just wait around. As it turns out, the PET showed that there were 2 more tumors (smaller) in the same area, nothing in the lymph nodes and a questionable area in the opposite lung. Nothing in other organs, thankfully.

In the meantime, the same day I got the PET scan I saw a thoracic surgeon (w/cancer specialty) who scheduled a mediastinoscopy/bronchoscopy to take a close look at the lung lymph nodes and biopsies. Again, thankfully, the nodes appear clear.

I am now scheduled for a lobectomy of my upper right lung in 4 days. At this point, we still don't definitively know if I have primary lung cancer or oligometastatic breast cancer. The surgery will provide the tissue to determine that.

My oncologist's office appealed the denial of coverage for the PET, but even on peer to peer review, they continued to deny. The assistant at the oncologist's office could not tell me the basis on which they made the denial. I have  heard nothing directly from the insurance company, yet, so I'm going to call them myself on Monday. So here are my questions: Have any of you had a similar experience with a PET denial during a cancer diagnosis? If so, what did you do? Were you able to get coverage on appeal? In general, any of you who have dealt with denials of various tests, what do you think is the best way to deal with the insurance company to effectively communicate and get a denial overturned?

I'm not even going to mention my feelings when I heard the denial, as I'm sure all of you have experienced those same negative feelings and I don't choose to dwell on them--not what I need to focus on right now.

Thanks for any advice/encouragement you can give to me on this issue.

Nancy