To sum it up: not bone cancer.
Detailed information: The bony growth is the type normally found in larger breeds of dogs. The weight on such a thin-ish joint causes rubbing/grinding and can (an does) cause a bone spur or bony growth, causing pain and limping. Loki is very large cat. Not overweight, just BIG.
The good news is it can be treated by anti-inflammatories. If the spur rebuilds up, it will be removed again. And as a bonus, treatment is covered by pet insurance.