Not a vet, but my wife and I have had two cats (like small dogs, but more selfish and usually softer) - die from cancer.
One had a tumor similar to yours. We opted for surgery to remove it, and the cat died on the operating table. It ended up costing a couple of grand, which was a big hit for two starving graduate students.
The other developed a tumor closer to the surface (outside the ribcage), and we did everything for that cat. Surgery, followed by daily radiation treatments for a month (at University of Missouri's vet school). The cancer came back, and we tried chemo. It still ate him alive.
(don't even ask how much we spent).
If that is a cancerous tumor, I would spoil the shit out of the animal, and give it all the favorite foods, treats, etc. - until it starts to show ANY signs of suffering, and then I would end it.
I hope it turns out to be something manageable, but if not, you have my best wishes and positive thoughts.