X-ray of my 15 y/o girl, vet just said “massive abnormality”

I noticed that Andy was laboring to breathe last weekend, started sleeping in strange places and wasn’t at ornery as usual. Then I realized just how much weight she’d lost, I felt bones I hadn’t felt in a decade or more. Got into the vet yesterday, and this was the Xray they took. He said maybe cancer, maybe a fungal infection but that it was constricting her breathing. He also said a biopsy would tell us for sure but at her age there wasn’t much we could do. She was given a steroid shot just in case it was a fungal infection (maybe I’m getting the exact details wrong, I was pretty distraught) and said we’d reassess in a week or two.

Does anyone have any input on what this might be? Is she in pain? She’s barely eating and I’m just heartbroken and looking for advice.