Hi gdrawmike,
Bone cancers are not particularly breed selective, and most cancers of bone tend to center near a joint, not within it, and tend also to remain on one side of the joint, not cross it. There would be considerable pain associated with the swelling, as well.
You say the vets 'seem to think'? It still is undetermined? Not all swellings of joints are cancer, probably most are not. At least an x-ray should be taken. Possibly 'scope the knee for a look and biopsy?
Vets nowadays are doing things similar to our human counterparts for cancers. As with people, the longer it goes undiagnosed, the greater the chance for spread.
It takes more than symptoms and signs to confirm a cancer, when tests are available to do so.
Swollen knees after having had surgery previously should make a vet consider the symptoms related to the old injury first. Maybe there is a reluctance to admit the knee has a problem since it was 'fixed' several years ago? I'm not trying to diagnose this episode, rather just to offer alternative explanations. There is always the 'second opinion' alternative, too.
I hope this has helped. Maybe just insight? Best wishes.
Please check just one ACCEPT box. Feedback is nice, too. Medicine comes in doses of Science, Art, and Miracle, given as needed.